Tribune was a title given to members of the Ruling Tribunal of the Aquans on the planet Argo. The newest were known as the junior tribune, the elders where known as the senior tribune, and the leader was known as the High Tribune. (TAS: "The Ambergris Element")
In 2269, Cadmar and Cheeron were among the senior tribunes, while Lemus, Nephro, and Rila were Junior Tribunes. The two groups held differing views, where the seniors believed that the juniors always wanted to change the ancient records. (TAS: "The Ambergris Element")
Though not explicitly stated in dialog, the script notes made it more evident that the division of younger tribunes that were sitting together, and later involved with helping Kirk and Spock, were in fact the Junior Tribune. The fact that Cadmar spoke of the Junior tribunes as if he was not one of them, combined with that fact that there was only one High Tribune, entertains the notion of a Senior Tribune, which was only otherwise referenced in the episode's script.