From my short time as a law clerk I believe it is a Summons and a Complaint that you would get if someone was suing you. They would send you a copy of their official Complaint and then have you served a Summons to appear in court on that complaint.

A Subpoena would be issued if they wanted a document or testamony from you. You could get a Subpoena to appear as a witness or for a deposition or to produce a document or other evidence that you might have in your posession. For example, we, on occasion would Subpoena a car dealership or the Secretary of State for Title records on a car, or the phone company for copies of someone's phone records.

The question then would be, did Anthony get a visit from the Sheriff or an official Process Server with a Summons and a Complaint or did the other party say I am going to sue you for $100,000.