Cal. Gov. Code § 56335
In each county containing two or more cities, regular and alternate city members to the commission shall be appointed by the city selection committee organized in the county pursuant to and in the manner provided in Article 11 (commencing with Section 50270) of Chapter 1 of Part 1 of Division 1. Regular members of the commission shall be appointed by the city selection committee pursuant to Sections 56325, 56326, and 56327.
The city selection committee shall appoint one alternate member to the commission in the same manner as it appoints a regular member. If one of the regular city members is absent from a commission meeting, or disqualifies himself or herself from participating in a meeting, the alternate member may serve and vote in place of that regular city member for that meeting.
Except in the case of a member appointed pursuant to subdivision (d) of Section 56326 or subdivision (b) of Section 56327, a city selection committee, may, at the time it appoints a member or alternate, provide that the member or alternate is disqualified from voting on proposals affecting the city which the member or alternate represents.
If the office of a regular city member becomes vacant, the alternate member may serve and vote in place of the former regular city member until the appointment and qualification of a regular city member to fill the vacancy.
Ca. Gov. Code § 56335