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Ad testificandum means to appear to testify. It is a term used to refer to an order issued by a court to appear to give testimony and may be directed to a prisoner through a writ of habeus corpus ad testificandum or to a civilian through a subpoena ad testificandum.
A writ of habeas corpus orders the custodian of an individual in custody to produce the individual before the court to make an inquiry concerning his or her detention, to appear for prosecution (ad prosequendum) or to appear to testify (ad testificandum). State courts may issue such writs to prisoner custodians to produce federal prisoners.