From the Summary: "The U.S. Constitution grants Congress the 'power of the purse' by prohibiting expenditures 'but in Consequence of Appropriations made by Law.' As a result, legislation to provide for government expenditures must adhere to the same requirements and conditions imposed on the lawmaking process as any other measure. [...] For discretionary spending, the exercise of this authority has resulted in the formalization of a process in which funding decisions are made in a two-step process, in which separate legislation to establish or continue federal agencies, programs, policies, projects, or activities is presumed to be enacted first, and legislation that provides funding is presumed to follow. In order for this two-step process to work, congressional rules therefore distinguish between legislation that addresses questions of policy and that which addresses questions of funding and encourage their separate consideration. In common usage, the terms used to describe these types of measures are 'authorizations' and 'appropriations', respectively. [...] This report provides an analysis of the relation of authorizations and appropriations, the impact of this distinction on the consideration of appropriations measures, and its significance for understanding how appropriations and other legislation work in conjunction to determine how agencies may spend appropriated funds."
CRS Report for Congress, R46497
Congressional Research Service: https://crsreports.congress.gov/