From the Document: "The primary mechanism for the Secretary of Defense to request, allocate, track, and expend DOD funds is known as the Planning, Programing, Budgeting, and Execution (PPBE) process. The 'Future Years Defense Program' (FYDP) is an annually compiled summary of the forces, resources, and programs associated with DOD operations. The FYDP is typically completed during the 'programming' phase of the PPBE process, and is generally updated during the 'budgeting' phase to reflect DOD's final funding decisions presented in the annual Presidential Budget Request (PBR). [...] Formally approved by the Secretary of Defense, the FYDP projects DOD funding, manpower, and force structure needs over a 5-year period. The FYDP depicts the allocation of defense resources in the two previous fiscal years and the current fiscal year and it provides force structure estimates for eight years. Section 221 of Title 10, United States Code, requires the Secretary of Defense to submit the FYDP to Congress in conjunction with the President's annual budget request. The FYDP is generally submitted to the congressional committees with jurisdiction over defense matters."
CRS In Focus, IF10831
Congressional Research Service: https://crsreports.congress.gov/