Multiple realities
(covers information from several alternate timelines)

A liaison was an individual who acted as an intermediary between two parties. This person could be officially designated as a liaison officer.

In 2268 while recording an entry in the ship's log, Hikaru Sulu described Ensign Pavel Chekov as the liaison between Captain James T. Kirk and the USS Enterprise. (TOS: "Spock's Brain")

In 2366, Captain Rachel Garrett made Lieutenant Richard Castillo the liaison between the USS Enterprise-C and the USS Enterprise-D. (TNG: "Yesterday's Enterprise")

In 2367, Hayne demanded to Jean-Luc Picard that Ishara Yar be their liaison between the Enterprise-D and his coalition. (TNG: "Legacy")