California Code, Government Code - GOV § 56331

When appointing a public member pursuant to Sections 56325, 56326, 56326.5, 56327, 56328, 56328.5, and 56329, the commission may also appoint one alternate public member who may serve and vote in place of a regular public member who is absent or who disqualifies himself or herself from participating in a meeting of the commission.The public member and the alternate public member shall be residents of the county of the appointing commission.

If the office of a regular public member becomes vacant, the alternate member may serve and vote in place of the former regular public member until the appointment and qualification of a regular public member to fill the vacancy.

No person appointed as a public member or alternate public member pursuant to this chapter shall be an officer or employee of the county or any city or district with territory in the county, provided, however, that any officer or employee serving on January 1, 1994, may complete the term for which he or she was appointed.