From the Document: "The FY2021 President's budget request includes $753.5 billion in budget authority for national defense. National defense is one of 20 major functions used by the Office of Management and Budget (OMB) to aggregate budget data―and the largest in terms of discretionary spending. The national defense budget function (identified by the numerical notation 050) comprises three subfunctions: Department of Defense (DOD)-Military (051); atomic energy defense activities primarily of the Department of Energy (053); and other defense-related activities (054), such as FBI counterintelligence activities."
CRS Insight, IN11224
Congressional Research Service: https://crsreports.congress.gov/