Auxiliary Boiler is Auxiliary Boiler thats it.

is can a main verb??

An Auxiliary force is a group affiliated with, but not part of, a military or police organization. In some cases, auxiliaries are armed forces operating in the same manner as regular soldiers. Most often, auxiliaries are civilian volunteer components supporting the main police or military force.

Auxiliary sound refers to auxiliary interface connector where one can connect external sound devices.

Underground is the element of an insurgent organization that conducts operations in areas denied to other elements.