https://en.wikipedia.org/wiki/Diplomat
"A diplomat is a person appointed by a state to conduct diplomacy with one or more other states or international organizations. The main functions of diplomats are: representation and protection of the interests and nationals of the sending state; initiation and facilitation of strategic agreements; treaties and conventions; promotion of information; trade and commerce; technology; and friendly relations. Seasoned diplomats of international repute are used in international organisations (e.g. United Nations) as well as multinational companies for their experience in management and negotiating skills. Diplomats are members of foreign services and diplomatic corps of various nations of the World."
There is really not much difference between a diplomat in GGE and in real life, just to give you some perspective. However since this is a game and that most alliances operate differently compared to each other, there are really no rules bound to this rank. Your own alliance is free to give this position it's own meaning if you so wish.