《太平广记·卷一百八十七·职官》
唐玄宗开元以前，皇帝亲自任命的官员为使臣，不是这样的不能称使。
开元已前，于外则命使臣，否则止。
Before the beginning of Tang Xuanzong's Kaiyuan era, the officials personally appointed by the emperor were envoys, otherwise, they were not envoys.