A Starfleet Special Task Force was an elite special forces military unit task force consisting of starship vessels present within the Starfleet that was tasked with safeguarding the Federation against serious internal threats. Whenever there is a violation of the Federation Charter by a member or whenever there exists strife between Federation states; it is the SSTF that is called to deal with the situation. It does this through a mixture of diplomacy, military force and intelligence gathering.
Captains within the SSTF are known to be completely loyal to the Federation and very idealistic. In fact, its been known for such officers to renounce ties to their home world and claim that the Federation is their home. Though not a legally binding statement, it does demonstrate the impartiality such officers possess and shows the vital qualities they have within Starfleet.
Its been noted that for twenty years, there has been no dissension within the Federation and that Captains of the SSTF have become almost indistinguishable from other Starfleet officers, except for the fact that members of the SSTF tend to read and file more reports. Despite this, selection by the SSTF is considered one of the highest honors available to a Starfleet officer. (TOS video game: Starfleet Command)