En este contexto U.S. es "United States Supreme Court Reports"

Wikièdia:
The Court's opinions are published in three stages. First, a slip opinion is made available on the Court's web site and through other outlets. Next, several opinions and lists of the court's orders are bound together in paperback form, called a preliminary print of United States Reports, the official series of books in which the final version of the Court's opinions appears. About a year after the preliminary prints are issued, a final bound volume of U.S. Reports is issued. The individual volumes of U.S. Reports are numbered so that users may cite this set of reports (or a competing version published by another commercial legal publisher but containing parallel citations) to allow those who read their pleadings and other briefs to find the cases quickly and easily.