The order that we should send a few people to help the other groups was received yesterday.我們應派幾個人去幫別的幾個小組的命令昨天收到了。(同位語從句,是對order的具體解釋,that雖不作成分,但不能省略。)
The order that we received yesterday was that we should send a few people to help the other groups.我們昨天收到的命令是我們應該派幾個人去幫助別的幾個小組。(定語從句,是名詞order的修飾語,that在從句中作received的賓語,可以省略。)