NASA in the Apollo decades (1960s to 1970s) used a type of planning that was represented in the titles of documents you'll find on NTRS, with esoteric titles such as “Phase A Shuttle Studies” etc.

What do those titles mean?

Thanks to a congressional document – FUTURE NATIONAL SPACE OBJECTIVES – STAFF STUDY FOR THE SUBCOMMITTEE ON NASA OVERSIGHT OF THE COMMITTEE ON SCIENCE AND ASTRONAUTICS U.S. HOUSE OF REPRESENTATIVES; EIGHTY-NINTH CONGRESS, SECOND SESSION – we know.

In cleaned up terms, they mean...

NASA PHASED PROJECT PLANNING

PHASE A – Advanced Studies – Analysis of Concepts and Feasibility – In this phase, the technical means available for satisfying the program objectives as defined by NASA are analyzed. A variety of alternative approaches and concepts that can satisfy these objectives are also studied.

PHASE B – Project Definition – Development of Single Plan – In this phase, a single project plan is selected from Phase A studies and is subject to intensive study and development.

PHASE C – Preliminary Design – Preliminary Hardware Design and Final Planning – Critical hardware is designed and tested via breadboards and the final project plan is developed.

PHASE D – Development & Operations – Full Design, Fabrication, Test and Operation – In this phase, all hardware is developed to it's final operational state, fabricated, tested and used operationally.