The Trill body that oversees the joining of hosts and symbionts and once concealed how many Trill were actually able to be joined.
The Symbiosis Commission controls which Trill may become hosts to a symbiont. By claiming that only a small fraction of the population was suitable, it preserved the prestige and scarcity of joining.
In 2371 Jadzia Dax was taken to Trill with neurological symptoms that Doctor Renhol linked to a suppressed host. The crew learned that Joran Belar, a murderer, had once been joined to Dax and that the Commission had covered it up. The truth also revealed that roughly half of all Trill could be joined, a secret the Commission had kept hidden.