I think it's a distinction without a difference. In this context, where this is no pending legal proceeding, a subpoena is just a request with no legal authority. The function of a real subpoena is to punish the recipient for failing to respond (i.e. held in contempt of court), because the request is made on behalf of the court and the court has the authority to compel production. Here, where there's no court case it's not really possible to have a real subpoena. I could equally issue a subpoena in the authority of my own name, and it would have just as much power (none).