31 U.S.C. § 1551 — Definitions; applicability of subchapter

(a) Definitions. (1) At the end of a fiscal year, an “obligated balance” is the account’s unliquidated obligations minus amounts collectible as repayments to the appropriation. (2) An “unobligated balance” is the difference between the obligated balance and the total unexpended balance. (3) A “fixed appropriation account” is an account available for obligations for a definite period. (b) Applicability. The spending-availability limits in this subchapter apply to all appropriations unless another law specifically extends a particular account’s availability and: (1) identifies that account; (2) says it may record, adjust, and liquidate obligations properly chargeable to it; and (3) extends the spending availability of its obligated balances. (c) Exceptions. This subchapter does not apply to appropriations for (1) the District of Columbia government or (2) disbursement by the Secretary of the Senate or the Chief Administrative Officer of the House of Representatives.