Deciders were the leading officials of Alzarian society. There were always three deciders, one being the head.
Decider Draith was the head decider for some time. At some point before the Fourth Doctor arrived on Alzarius, the other two Deciders were Nefred and Tannek. (PROSE: A Boy's Tale)
Later, Garif replaced Tannek. When Draith died, he was replaced as Head Decider by Nefred. Login was then hired to make up the third alongside Garif. The head decider was the only person with access to the System Files. (TV: Full Circle)