From the Introduction: "Reconciliation is a process established under Section 310 of the Congressional Budget Act of 1974 (P.L. 93-344, as amended). The purpose of reconciliation is to change substantive law so that revenue and mandatory spending levels are brought into line with budget resolution policies. [...] Between 1980 and 1985, the reconciliation legislation contained many provisions that were extraneous to the purpose of reducing the deficit. The reconciliation submissions of committees included provisions that had no budgetary effect, that increased spending or reduced revenues, or that violated another committee's jurisdiction. In 1985 and 1986, the Senate adopted the Byrd rule (named after its principal sponsor, Senator Robert C. Byrd) as a means of curbing these practices. Initially, the rule consisted of two components, involving a provision in a reconciliation act and a Senate resolution. The Byrd rule has been modified several times over the years. The purpose of this report is to briefly recount the legislative history of the Byrd rule, summarize its current features, and describe its implementation from its inception through the present."
CRS Report for Congress, RL30862
Congressional Research Service: https://crsreports.congress.gov/