"Federal spending is divided into three broad categories: discretionary spending, mandatory spending, and net interest. Those categories are procedural--that is, how Congress provides or has provided spending authority differs among those categories. The Budget Enforcement Act of 1990 (BEA; P.L. 101-508) provides a statutory definition for those terms, which are therefore referred to as BEA categories. To large extent, however, the type of spending differs across those categories. Discretionary spending, by and large, funds operations of federal agencies. Net interest spending is the government's interest payments on debt held by the public, offset by interest income that the government receives.Major entitlement programs such as Social Security, Medicare, and Medicaid make up the bulk of mandatory spending. Other mandatory spending funds various income support programs, including Supplemental Security Income (SSI), unemployment insurance, and the Supplemental Nutrition Assistance Program (SNAP), as well as federal employee and military retirement and some veterans' benefits.In recent decades, mandatory spending has been the largest component of the federal budget."
CRS Report for Congress, R44641
Congressional Research Service: https://crsreports.congress.gov/