Reserves are the soldiers without nation and name. They appear in Tom Clancy's Rainbow Six and its expansion: Eagle Watch. Their flag is that of the UN, meaning that their serve under the UN and NATO council.
Reserves have all specialties, because they are trained to every possible situation that a counter terrorism agent can encounter. They have all specialties too because they have access to all kind of training and counter terrorism agencies to train them. This access is granted by the UN council.