《汉书·志·艺文志》
是说使者应当权衡事情，见机行事，接受使命但不接受言辞，这是他们的长处。
言其当权事制宜，受命而不受辞。此其所长也。
That is to say, the messenger should weigh the situation and act accordingly, accepting the task but not accepting the words, which is their strength.