An officer is always commissioned by an elected official or officials. The commissions of US military officers are recommended by the President (through the Secretary of Defense) and approved by Congress. In the Missouri National Guard, the legislature approves commissions recommended by or on behalf of the Governor. In the Auxiliary, the Sheriff is our elected official and must personally approve commissions because it is his authority which is being delegated on behalf of his constituents.
The Commander or the Chief of Staff recommends officer candidates to the Sheriff which have been vetted by our established procedures. Let's look at the standard format for officer recommendations:

In accordance with your request to organize a volunteer auxiliary under the Lawrence County Sheriff's Office, and in conjunction with the Mission Statement and Organizing Document of this date, the following list of officer candidates is submitted for approval.

...

I hereby solemnly affirm that these candidates are, to the best of my knowledge and belief: honorable citizens, exemplary members of the community, willing of faithful service, not judicially barred from service under arms, and shall be a credit to both the Office of Sheriff and the LCS Auxiliary; attesting on my own honor to the same.

 

Straight off, we see the source of our authority in recommending officers: the Sheriff has made a request to organize an auxiliary. The auxiliary was embodied in its organizing documents (which are periodically approved by the Sheriff), and, in accordance with those documents, we are submitting candidates for the Sheriff's approval.
The last part defines the character required of officers and follows a traditional format. The officers exist to fulfill an office, a "a position of authority, trust, or service, typically one of a public nature" (definition 5), given to the Sheriff by the people and delegated to us in turn. We discussed the origins and authority of the Office of the Sheriff in CLA-I.