What is the official position of the messenger in Song Dynasty?

Well, the so-called emissary refers to the emissary of the son of heaven, usually a eunuch. You can say that he is an imperial envoy, but he usually doesn't care about anything but simply reads the imperial edict. Of course, some dynasties, such as Song Dynasty, sent eunuchs to supervise the military command of military commanders, although most of them failed.