California Code, Elections Code - ELEC § 3108

(1) The military or overseas voter is released from service after the closing date of registration for an election, returns to the county of his or her residence, and is not a registered voter. To register, the elector shall furnish documentary proof that he or she was released from service after the closing date of registration for the election.

(2) The military or overseas voter is required to move under official active duty military orders after the closing date of registration. To register, the elector shall furnish a copy of his or her official military orders.