California Code, Revenue and Taxation Code - RTC § 1751

(a) “Multijurisdictional assessment appeals board” or “board” means a board established by two or more counties pursuant to this chapter meeting as an assessment appeals board on behalf of the participating counties.

(b) “Participating county” means a county that has entered into an agreement, by enactment of an ordinance pursuant to Section 1752.1, to create a multijurisdictional assessment appeals board for the purpose of equalizing the valuation of assessed property.