Final answer:
When the U.S. Supreme Court hears a case appealed from a circuit court, it is utilizing its appellate jurisdiction to review and potentially overturn lower court decisions. Most of the Supreme Court's caseload is appellate, with original jurisdiction being relatively rare and used only in specific instances.
Step-by-step explanation:
When the U.S. Supreme Court agrees to hear a case appealed to it from a circuit court, it is exercising its appellate jurisdiction. This means the Supreme Court has the power to review decisions of lower courts and potentially overturn them. While the Supreme Court has both original and appellate jurisdiction, the bulk of its cases are heard under its appellate jurisdiction. Original jurisdiction applies when the Supreme Court is the first to hear a case, which is limited to cases involving ambassadors, public ministers, consuls, or where a state is a party. Every other federal issue that comes before the Supreme Court does so on appeal, making appellate jurisdiction the main function of the Court as the "court of last resort" in the United States judicial system.