Cal. Public Utilities Code § 102351

(a) The retail transactions and use tax ordinance shall provide for a rate of one-eighth of 1 percent or a multiple thereof. The ordinance shall provide for the tax to be imposed on the entirety of, or a portion of, the incorporated and unincorporated territory within the boundaries of the district, as defined in Section 102052.5. The ordinance shall be subject to voter approval by the electors in those portions of the district to which the tax applies and shall be consistent with Section 102350. (b) The portion of the area of the board to which the tax would apply shall be determined by the board before the electors vote on the measure. (c) If the tax only applies to a portion of an area of the district, all of the following shall apply to the imposition of the tax: (1) The incorporated area of each city and of contiguous cities within the district shall be either wholly included within that portion or wholly excluded from that portion. For purposes of this paragraph, “contiguous cities” means two or more cities with shared borders. (2) The entire unincorporated area of the district shall either be wholly included within that portion or wholly excluded from that portion. (3) The revenues derived from the tax shall be spent within, or for the benefit of, the portion of the area of the district to which the tax applies, and shall be spent only on allowable transportation and transit infrastructure and services. (d) Any revenues derived from the tax shall supplement, and not supplant, other transportation revenues available to the portion of the area of the board to which the tax applies.