The Rapid Reaction Force, or RRF for short, are a UCN planet's first line of defence against invading enemies or internal ones. The RRF are part of the ISA Army. All ISA planets have their own RRF Units, ranging in skill and effectiveness based on what the local government is willing to pay. But no matter what the colony the RRF make up 15% of the Army on all these colonies. Since the ISA Army acted as both a police and a military force the RRF most likely acted as riot police, while the regular soldiers acted as patrol units.
RRF members are recruited for their natural leadership abilities as well as personal traits and characteristics that help them become example soldiers. As elite soldiers the RRF are expected to hold the first line of defence while other ISA soldiers organise more substansial defences. However, the UCN's opinion of what were effectively glorified colonial police was possibly overestimated when they expected them to face conventional threats. However there were some exceptions to this, especially by Captain Jan Templar, who later went on to form the ISA's elite Special Forces.
The RRF played a crucial role in defending Vekta against the Helghan invasion in the Second Extrasolar War, as they are the first called into combat. The casualties they sustained during the Vektan campaign were so great that the Vektan RRF were disbanded after the Helghast were forced off of the planet, with the few dozen survivors being transferred as commanding officers of the Legionnaires naval special forces unit in preparation for the invasion of Helghan.