In Paragangia, a person assigned to deal with a particular group of people on behalf of Paragangia, [such as an alien race or an independant community such as D'zeron] is known as a Negotiator.
In the time of Desmond Harris and Jasnon Scott-Harris, instead of having a military [which they don't need because they are protected by Paragangia] they have the Earth Alliance Negotiators Corps.
Jason and Desmond both have this title, Jason is a Negotiator for Paragangia in D'zeron. Desmond is a member of the Earth Negotiators Corps in Paragangia.
Matthias in Jason's time is in training at KSU to be in the Negotiators Corps, and will probably grow up work as a Negotiator on Earth Station Mexico. When he is hired as Jason's teaching assistant, he considers himself a negotiator for Jason, because he deals with the outside world for Jason, in a way similar to what negotiators do, and he is in training to become one.
in Desmond's time, Jonathan is the Negotiator for Paragan, which means he deals with people in person that Desmond does not want to deal with in person, so Desmond can keep his secret Identity.