From Halopedia, the Halo wiki

Auxiliaries were a type of warship employed by the naval forces of the Forerunner Ecumene. Within the Ecumene's naval classification system of orders of battle, auxiliaries (alongside their interdictor brethren) rated as mere sixth- or seventh-order hulls.[1]

The full scope of the auxiliaries' role in the Forerunner fleet is unknown, though their classification fairly far down the order system suggests they were not considered particularly powerful warships.