A subpoena is a United States legal process that a court uses to make a person provide things, such as documents, or to provide testimony in a hearing or a deposition. So I interpret the last sentence as meaning that the subpoena will be issued to Shepard, informing him that he is going to have to provide documents or testimony, and then afterwards, the arrangements of what he will be required to provide or where he will be required to provide testimony will follow. The sentence isn't very clear, though, so I am not sure if my interpretation is correct or not.