A witness summons is a document that can place a requirement on a witness to attend court. A witness summons is generally issued when there is a hostile witness or witnesses that are felt would not attend under their own free will. Witness summons are generally used for either of these things – if the person is a civilian they could be summonsed to court to give evidence or to secure a piece of evidence they may have – if they are an expert witness then they could be given a summons to produce certain documents/records.