Furthermore, after Ryoge no kan (a post outside the original Ritsuryo code created by Imperial edicts) was established, cho was generally used for official documents in relation to Ryoge no kan (cases in which the sender or the receiver, or both, were Ryoge no kan) because in some cases no hierarchical relationship existed among Ryoge no kan.
更に令外官が設置されるようになると、令外官には所管・被管の上下関係が存在しない場合があるため、原則的に令外官が関わる（差出人・宛先人の片方もしくは双方が令外官）の場合には牒が用いられていた。