A subpoena or witness summons is a writ issued by a government agency, most often a court, to compel testimony by a witness or production of evidence under a penalty for failure. There are two common types of subpoenas:subpoena ad testificandum orders a person to testify before the ordering authority or face punishment. subpoena duces tecum orders a person or organization to bring physical evidence before the ordering authority or face punishment. This is often used for requests to mail copies of documents to a requesting party or directly to a court.