The Fifteenth Army, which was also designated as Hook Nebula Command, functioned as a Sector Army within both the Republic and, subsequently, the Imperial military, with its main responsibility being the protection of the Sarin Oversector.
Throughout the Clone Wars conflict, the Fifteenth Army was conceived to serve predominantly as a reserve unit, with the capacity to reinforce troops engaged in battle along the Corellian Run, the Hydian Way, and the Rimma Trade Route.