In 2399, the location of Tellun was labeled on a Federation star chart that was in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. The system was in or near to Federation space. (PIC: "Maps and Legends")
In 2268, both civilizations possessed the capability of mutual destruction. In that year, both the Federation and the Klingon Empire considered the Tellun system to reside within their respective borders. The Federation sought an end to the internal conflict and served as a mediator between Elasians and Troyians, while the Klingons tried to subvert the peace efforts in the hopes of eventually gaining access to the vast Elasian dilithium deposits.
The USS Enterprise, a Federation starship, was assigned a secret diplomatic mission to facilitate the marriage of Elaan, the Dohlman of Elas, to the ruler of Troyius. The mission was interrupted by sabotage and an indecisive ship-to-ship engagement with a Klingon D7 class battle cruiser. Regardless of the obstacles, the mission was a success. (TOS: "Elaan of Troyius")
In 2328, this was the destination for passengers traveling from Earth Colony 2 via commercial transport. Mary Joy Macalintal, Michael Westmore, and Marion Westmore traveled there on the SS Kogin. (TNG-R: "Inheritance", okudagram)