ancillary

Subordinate; secondary; auxiliary.

[…] how easily he took all things along with him,—the persons, the opinions, and the day, and nature became ancillary to a man.

[E]very organ of the body, whatever function it may perform, must also perform the other four functions in an ancillary manner.

Something that serves an ancillary function, such as an easel for a painter.

Auxiliaries and ancillaries are comprehensive, and include a Westinghouse motor-driven recriprocating compressor used for locomotive braking and general service air, two rotary exhauster sets for train brakes when hauling passenger or fitted freight trains, and an oil-fired train heating boiler.

An auxiliary.