Use cases and functions¶

Summary. Use cases and functions are the middle layer of NASA's objective decomposition: objectives → characteristics and needs → use cases and functions → reference missions and elements. ADD Rev C's Appendix B lists 163 lunar use cases and 208 lunar functions (pp. 110–127), then 148 Mars use cases and 263 Mars functions (pp. 127–147). Its later sections map lunar functions to elements and list the functions no element yet performs. This page covers what Appendix B says, how to read an ID, and where the wiki's gap, Moon Base and element pages use the IDs. The full lists are on Lunar use cases, Lunar functions, Mars use cases and Mars functions.

What they are¶

NASA's definitions (ADD Rev C, p. 17):

Use cases: "Operations that would be executed to produce the desired needs and/or characteristics."
Functions: "Actions that an architecture would perform to complete the desired use case."

And in the method's own words: "Functions are services or actions that the exploration architecture would perform. Use cases describe operations that the architecture employs" (p. 17). Where they sit in the decomposition, with the p. 16 figure, is on Objective decomposition.

In practice:

A use case names an operation and where it happens. Example: "Conduct of crew extravehicular operations on the lunar surface" (UC-M-101 L).
A function names a capability. Example: "Enable crew lunar surface extravehicular activity in sunlit areas and non-PSRs" (FN-M-101 L).

This reading is the wiki's, from the two lists.

What Appendix B says about them (B.1, p. 109)¶

All quotations are from ADD Rev C, p. 109, checked against the PDF.

The tables. "The tables linked below capture the objective decomposition for lunar and Mars objectives, tracing from objectives through characteristics and needs to use case and functions, as described in Section 1." B.1.1 "Lunar Objective Decomposition" and B.1.2 "Mars Objective Decomposition" are links to NASA's architecture documents page (https://www.nasa.gov/moontomarsarchitecture-architecturedefinitiondocuments/; its source page). The wiki holds the two decomposition spreadsheets in sources/text/docs/. The lunar one is written up on Lunar objectives and Lunar function allocation (source page). It has no characteristics-and-needs layer, although its key lists sheets for one. The Mars one (source page) is on Mars use cases, Mars functions and Mars objectives. It lacks the same layer, and it allocates no functions to elements.
Support functions. "The objective decomposition also considers the existence of certain support functions — including power, communications, command and data handling, positioning, navigation, and timing — that are inherent to almost all spaceflight elements. These support functions need not be explicitly mapped within every single objective; they are documented, but they are mapped to other objectives that more specifically address the functional area."
Numbering. A boxed note: "The lunar and Mars objective decompositions follow independent numbering schema; the numbering for Mars use cases and functions does not follow sequentially from the lunar decomposition."
"Deep space" and "Mars vicinity". A second boxed note: "The objective decomposition uses the term 'deep space and/or Mars vicinity.' 'Deep space' is the vast region of space that extends to interplanetary space, including Mars and beyond. The term 'Mars vicinity' refers to the region of space around the Mars system, including Mars orbit. As the Mars architecture continues to mature, it will add information about where and how certain use cases and functions are performed, including specific trajectories and orbits. Currently, 'Mars vicinity' is the highest appropriate level of specificity. Appendix E captures additional terms and definitions." Appendix E is the data-gap catalog. Terms are in Appendix F, "Terminology" (contents, p. 7) (open question 21). Appendix F defines "Deep Space Environments", "Cislunar Space" and "Earth Vicinity" but not "Mars vicinity" (pp. 295–296; Glossary).

What Appendix B holds¶

From the contents (pp. 6–7) and the pages themselves (all of Appendix B is read):

Section	Pages	What it is	In the wiki
B.1 Objective Decomposition	109	The text above; links to the decomposition tables	this page
B.2.1 List of Lunar Use Cases	110–117	163 use cases, ID and title only	Lunar use cases
B.2.2 List of Lunar Functions	117–127	208 functions, ID and title only	Lunar functions
B.2.3 List of Mars Use Cases	127–134	148 use cases, ID and title only	Mars use cases
B.2.4 List of Mars Functions	135–147	263 functions, ID and title only	Mars functions
B.3 Element function mappings	148–165	Which lunar functions each of the 15 elements performs, one table per element and segment (HLR, FE). "Human Lunar Return segment elements form the basis for satisfying functional needs" (p. 148)	Each element page, under "Functions (ADD Appendix B.3)". Compared with the lunar spreadsheet on Lunar function allocation
B.4 Utilization payloads and equipment function mappings	166	"Equipment" only: 1 function in HLR, 8 in FE. "Science/research payloads and technology demonstrations are forward work"	Lunar function allocation
B.5 Unallocated Functions by Use Case	167–184 (p. 185 blank)	79 use cases with the functions no element performs: 4 in HLR (all FN-C-205 L), 75 in FE (77 functions)	Lunar function allocation, in full and against the spreadsheet's 76 "Unallocated"

The B.2 tables carry only an ID and a title: no use case–function link, no objective, no segment, no priority and no element. On the rendered page the use-case tables have green headers and the function tables orange ones. The counts are the wiki's, made from the lists by ID.

Order. All four lists run through the letters in the order T, G, H, X, L, P, M, C, D, A, I, U (the Mars use-case list has no L). That isn't the order of the ADD's p. 40 table (A, C, D, H, X, G, I, L, M, P, T, U), and the ADD doesn't explain it. The lunar decomposition spreadsheet's key numbers the letters 1 to 12 in exactly this order, from 1 Transportation (T) to 12 Utilization (U) (source page).

How to read an ID¶

Take FN-P-101 L, "Generate power in the south pole region on the lunar surface" (p. 121).

Part	Example	Meaning	Basis
Prefix	`FN`	function; `UC` is a use case	The column headings "FN ID" and "UC ID" (pp. 110, 117)
Letter	`P`	the sub-architecture, here Power Systems	The p. 40 "Decomposition Abbreviation" (Sub-architectures). Appendix B doesn't spell it out, but the lunar decomposition spreadsheet's key pairs each letter with a sub-architecture name ("6 Power P")
Number	`101`	a named group (the hundreds) and a place in it; for P, 100 is "Generation"	The lunar decomposition key names the groups for each letter (source page). The place within the group is the wiki's reading.
Suffix	`L`	the lunar list; `M` is the Mars list	Every ID in the "Lunar Use Case" and "Lunar Function" tables ends in L, and every ID in the "Mars Use Case" and "Mars Function" tables ends in M (pp. 110–147). B.1's note says the two "follow independent numbering schema" (p. 109).

The same number means different things on the Moon and on Mars. FN-T-101 L is "Transport crew from Earth to cislunar space" (p. 117), but FN-T-101 M is "Staging of crewed Mars mission(s) in Earth vicinity" (p. 135). Always keep the suffix.

The first digit is a named group. The lists show it, and the lunar decomposition spreadsheet's key confirms it and names the groups. For transportation the key gives 100 "Crew Transportation", 200 "Cargo Transportation", 300 "Cargo Conditioning", 400 "Landing" and 500 "Contingency". So:

FN-T-1xx L: crew transport, habitation in transit, abort
FN-T-2xx L: cargo transport
FN-T-3xx L: conditioning samples in transit
FN-T-4xx L: landing
UC-T-501 L, "Testing, contingency planning, and edge-case analyses of in-space systems", is the only entry in 500

Other groups: FN-M-5xx L is "Cargo Repositioining" (as printed), FN-U-4xx L "Store Samples" and FN-U-5xx L "Propellant Demonstration". The full list of groups is on the source page. It also gives the one ID outside the groups, FN-M-701 L (the key has no M 700 group), and some titles that don't match their group's name. Some hundreds have no use cases, for example UC-U-4xx L (Lunar use cases).

The Mars key is the lunar key, group for group, but the Mars lists use three groups it doesn't name: G 400 (servicing, inspection and anomaly handling, e.g. FN-G-408 M), G 500 (off-nominal response and contamination control, e.g. UC-G-501 M) and C 300 (command and control and uncrewed modes, e.g. FN-C-307 M). That is 7 use cases and 24 functions, by the wiki's count (Mars source page). The Mars lists also have no M 100 "EVA" functions and no L use cases. The 2024 Mars key names all three: G 400 "Asset Health Management", G 500 "Contingency (Miscellaneous)" and C 300 "Command & Control" (2024 Mars table).

A third kind of ID: characteristics and needs. The 2024 mapping tables also number the characteristics and needs, the step between objectives and use cases, in the same pattern: CN-T-108 L is letter, number and suffix. The numbers are their own, so CN-T-108 L is not UC-T-108 L. The C&Ns are in neither the ADD nor the December 2025 workbooks; the lists are on Lunar characteristics and needs and Mars characteristics and needs.

Where the IDs are used¶

Technology gaps. Each gap's "Architecture Traceability: Use Cases & Functions" field lists pairs such as UC-L-201 L -- FN-L-101 L. It can also give a use case with "All FN", or a use case alone (tech gaps spreadsheet). ADD Appendix D prints the same pairs (pp. 199–260; one Mars difference under #1107, open question 37). The wiki's derived counts:
- 61 of the 163 lunar use cases are cited by a tech gap, and 59 of the 148 Mars use cases.
- 41 of the 208 lunar functions are named by one, and 40 of the 263 Mars functions.
The reverse indexes are on the four list pages. Three Mars pairings in the gap tables are not pairings the Mars decomposition makes (open question 60).
Moon Base Phase 1. The Users Guide's 31 Phase 1 functional gaps are all lunar functions (Phase 1 functional gaps). Compared with the ADD's list:
- 19 match word for word
- 3 differ in wording
- 8 differ only in typography
- 1, "FN-P 102 L", carries the title of FN-P-202 L
Details on Lunar functions; open question 40.
Unallocated functions. The Users Guide defines functional gaps as "architecture functions that are either currently unallocated to any existing elements, or functions that need additional performance to be fully satisfied". "A full list of unallocated functions can be found in an appendix of the Architecture Definition Document" (Users Guide, p. 9). By title, that is B.5 "Unallocated Functions by Use Case" (p. 167). The match by title is the wiki's (ADD source page). B.5 holds 11 of the guide's 31 Phase 1 functional gaps; the other 20 are mapped to elements in B.3 (Phase 1 functional gaps; open question 65).
Elements. Each element one-pager names its "Functional Mappings" section in Appendix B.3 (Elements). The 2025 Architecture Update prints the functions of the two Rev C elements, the Lunar Utility Rover and the Lunar Nuclear Fission System (Update, pp. 5, 7). Every element page lists its B.3 functions. No Mars function is mapped to an element: for Humans to Mars that mapping "is forward work" (p. 38).
Revision history. Revision B.1 (April 2025) "Removed allocation of FN-T-103 L, 'transport crew from cislunar space to distributed sites outside of the south pole region on the lunar surface,' to UC-T-103 L, 'aggregation and physical assembly of spacecraft components in cislunar space'" (p. 4). Both titles match the B.2 lists, apart from the lower-case first letter. Revision B (December 2024) "Added Mars characteristics, needs, use cases, and functions for Transportation and Habitation, Mars Infrastructure, and Operations objectives", and Revision C "Added Mars use cases and functions for lunar and planetary science, heliophysics science, human and biological science, and physics and physical science objectives" (p. 4). Which Mars entries are new in Rev C can't be told without Rev B; the wiki doesn't hold it. Rev B's Mars "characteristics, needs" are not in the December 2025 Mars spreadsheet (open question 42).

What the lists don't tell you¶

Which functions serve which use case. B.2 gives two separate lists. The pairings appear in the decomposition spreadsheets (B.1), in B.5's "by use case" layout, and in the gap tables. This is why "All FN" in a gap table can't be expanded from B.2. The two spreadsheets' pairings expand the "All FN" entries on the gaps index, lunar and Mars. The Mars pairings are also columns on Mars use cases and Mars functions.
Which objective a use case or function serves. That is in the decomposition spreadsheets: Lunar objectives and Mars objectives.
Which element performs a function, or whether any does. For lunar functions that is in B.3 and B.5, and in the lunar spreadsheet's allocation sheets, marked "FORWARD WORK". The two agree closely (Lunar function allocation). No source maps Mars functions to elements.
Priority or timing. The lists carry no rank, segment or date.

Lunar use cases · Lunar functions · Mars use cases · Mars functions · Objective decomposition · Objectives: the ten goals · Lunar objectives · Mars objectives · Lunar function allocation · Sub-architectures · Segments · Elements · Gaps index · Moon Base Phase 1 functional gaps · ADD Rev C source page

Sources¶

ADD Rev C, pp. 4, 6–7, 16–17, 38, 40, 109–185 (PDF checked on pp. 109–127, 130–134, 137–139, 141, 145, 147, 158, 166, 169, 175 and 176) · Tech gaps spreadsheet, "Architecture Traceability: Use Cases & Functions" column · Users Guide, pp. 8–10 · 2025 Architecture Update, pp. 5, 7 · Lunar objective decomposition spreadsheet, key sheet and "FE+ Functions to Use Cases" · Mars objective decomposition spreadsheet, key sheet