The public voucher must be accompanied by a subpoena or invitational orders (Joint Travel Regulations, vol. 2, chap. 6), and by a certified copy of the order appointing the court-martial, court of inquiry, or investigation. If, however, a deposition is taken before charges are referred for trial, the fees and mileage of the witness concerned shall be paid by the disbursing officer at or near the place where the deposition is taken upon presentation of a public voucher, properly completed as hereinbefore prescribed, and accompanied by an order from the officer who authorized the taking of the deposition, subscribed by him and directing the disbursing officer to pay to the witness the fees and mileage supported by the public voucher. When the civilian witness testifies outside the United States, its territories and possessions, the public voucher must be accompanied by a certified copy of the order appointing the court-martial, court of inquiry, or investigation, and by an order from the convening authority or appointing authority, subscribed by him and directing the disbursing officer to pay to the witness the fees and mileage supported by the public voucher.
32 C.F.R. §719.138