Special envoy

A special envoy or special emissary was a diplomatic rank that was given a very specific mission.

In 2293, Spock was given the duties of "Federation Special Envoy" to the Klingon Empire, whereupon he conducted communications with Chancellor Gorkon. 

The half-Klingon K'Ehleyr was once granted the title while inside Federation space. Starfleet honored the moniker, because they viewed her as vital to the success of a mission regarding Klingons who still believed the was raging.

Though defining the title was difficult, the power-relationship between K'Ehleyr and Jean-Luc Picard was one of rough equality. He was obliged to put USS Enterprise (NCC-1701-D) at her disposal, and to let her lead the response to the situation. Nevertheless, he still had a say in the precise manner in which the mission was carried out. In particular, she had initially thought the mission would require Enterprise to simply kill the under-informed Klingons, but he successfully insisted on a change of tack. 

Ilon Tandro was a special envoy from Klaestron IV who was in charge of the extradition procedure of Jadzia Dax.