Every federal court has local rules that the court follows for that particular jurisdiction. Service of a lawsuit on a defendant can happen a number of different ways, with process server just being one of them. The most popular method (because it is the cheapest) is to simply serve the defendant via certified mail. The method of noting proper service on the docket varies by jurisdiction, but it often falls upon the plaintiff to actually provide proof of service to the court. So, essentially, the plaintiff shows the court the receipt.