"The Trump Administration's initial FY2018 budget request, released on May 23, 2017, included a total of $677.1 billion for the national defense budget function (Budget Function 050), which encompasses all defense-related activities of the federal government. Of that amount, $659.8 billion was for appropriation accounts for which authorization is provided in the annual National Defense Authorization Act (NDAA). The remainder of the request was either for mandatory funds not requiring annual authorization or for discretionary funds outside the scope of the NDAA. That initial Administration request included $595.3 billion in discretionary funding for the socalled base budget, that is, funds intended to pay for activities that the Department of Defense (DOD) and other national defense-related agencies would pursue even if U.S. forces were not engaged in contingency operations in Afghanistan, Iraq, Syria, and elsewhere. The remaining $64.6 billion of the request, formally designated as funding for Overseas Contingency Operations (OCO), would fund the incremental cost of those ongoing operations as well as any other DOD costs that Congress and the President agree to so designate."
CRS Report for Congress, R45013