Multiple realities(covers information from several alternate timelines)
The Starfleet Diplomatic Corps was a department within Starfleet.
The movement of personnel in and out of this corps was governed by the Starfleet Transfer Regulations. (TNG: "The Measure Of A Man", okudagram)
In the 2260s of the alternate reality, information on this corps was located in the Federation database. (Star Trek Beyond)
This corps was mentioned in a list of Starfleet agencies, which appeared to the left of the personnel file for Balthazar Edison. 
This corps was first mentioned in the extended version of "The Measure Of A Man".