The document proving that the other party was formally served. Generally, you only need a proof of service at the beginning of the case to prove the defendant received the complaint and summons according to the rules. If you serve the other party by certified mail, restricted delivery, return receipt the "green card" you get back signed by the defendant will be your proof of service. If you use a process server, the affidavit he or she gives you telling you when the defendant was served will be your proof of service.