Auxiliary, subsidiary, accessory, contributory, subservient, ancillary, adjuvant mean supplying aid or support.
Auxiliary may imply subordinate rank or position. It often suggests something kept in reserve.
Subsidiary stresses subordinate or inferior status or capacity, often to the obscuring or loss of the notion of supplying aid.
Accessory so strongly stresses association or accompaniment that the notion of assistance or support is often obscured or lost; thus, an accessory mineral is one present in a rock but not an essential constituent; a person accessory to a crime (as the hirer of an assassin or a receiver of stolen goods) need not actively participate in its commission (see also accessory n under CONFEDERATE).
Contributory stresses the assistance rather than the subordinate status of the assistant and usually implies the effecting of an end or result.
Subservient usually stresses the subordinate nature of the assistance. It may stress the importance or usefulness of the end it serves and the nature of its motive (as commendable self-subordination or a sense of order and due relation).
Ancillary more than the other terms stresses the intimacy of the assistance.
Adjuvant differs from auxiliary, its closest synonym, in attributing greater importance, more noticeable effectiveness, or a more definite influence to the thing so qualified; an adjuvant ingredient in a prescription often modifies the action of the major ingredient so as to make it effective.