Answer:
In civil cases, like personal injury, in order to subpoeana either a witness or a piece of evidence, you must file for the subpoena with the court, if it apporved it will be sent out with a process server, it must be signed and shown as being recieved, and then that person is required to hand over the requested evidence/show up to mediation hearings or to court. The attached article goes into more specifics regarding subpoena law.