From the web site: "United States Courts Opinions (USCOURTS) collection is a project between the U.S. Government Publishing Office (GPO) and the Administrative Office of the United States Courts (AOUSC) to provide public access to opinions from selected United States appellate, district, and bankruptcy courts. The content of this collection dates back to April 2004, though searchable electronic holdings for some courts may be incomplete for this earlier time period."
Officially named "United States Reports," these are cases adjudged in the U.S. Supreme Court. The October term of 1991 is the earliest available on this site. These volumes "contain not just opinions, but the full text, from cover through index, of bound volumes 502 et seq., including all of the opinions, orders, and other materials issued for the Court's 1991 Term and subsequent years. Additional volumes will be included here after they are published in print form." Only the print version is the final, official version.