"Discretionary spending is provided and controlled through appropriations acts, which fund many of the activities commonly associated with such federal government functions as running executive branch agencies, congressional offices and agencies, and international operations of the government. Essentially all spending on federal wages and salaries is discretionary. Spending can be measured by budget authority (BA; what agencies can legally obligate the government to pay) or outlays (disbursements from the U.S. Treasury). This report mostly discusses trends in outlays. Federal spending in fiscal year (FY) 2013 was just over a fifth (20.8%) of the U.S. economy, as measured as a share of gross domestic product (GDP), which is close to its average share since 1962. (Years denote federal fiscal years unless otherwise noted.) Discretionary spending accounted for 35% of total outlays in 2013 ($3,454 billion), well below mandatory spending's share (59% of outlays in 2013). Weak economic conditions in recent years as well as long-term demographic trends have increased spending on mandatory income support and retirement programs, while policy makers have acted to constrain the growth of discretionary spending. Net interest costs were 6.1% of federal outlays in 2013, but are projected to rise sharply."
CRS Report for Congress, RL34424