(covers information from several alternate timelines)
A liaison officer was usually a spokesperson who represented the interests of his or her government.
Kira Nerys was the Bajoran liaison officer on board the space station Deep Space 9 since 2369, representing the Bajoran Provisional Government and the Bajoran Militia when the Federation helped rebuild it. (Star Trek: Deep Space Nine)
Li Nalas was made Deep Space 9's liaison officer by Jaro Essa for a brief period in early 2370. It was shortly after his return from prison on Cardassia IV. (DS9: "The Homecoming", "The Circle", "The Siege")
A Cardassian liaison officer kept in close contact with Starfleet Admiral Kennelly during the crisis involving the Bajoran resistance leader Orta and his alleged attack on Solarion IV. (TNG: "Ensign Ro")
T'Rul served as a Romulan liaison officer assigned to supervise the operation of a cloaking device loaned to the Federation and installed on the USS Defiant in 2371. (DS9: "The Search, Part I", "The Search, Part II")