Cal. RTC § 19551

(a) The Franchise Tax Board may permit the Commissioner of Internal Revenue of the United States, other tax officials of this state, the Multistate Tax Commission, the proper officer of any state imposing an income tax or a tax measured by income or the authorized representative of that officer, or the tax officials of Mexico, if a reciprocal agreement exists, to inspect the income tax returns of any taxpayer, or may furnish to the commission, or the officer or the authorized representative thereof an abstract of the return or supply thereto information concerning any item of income contained in any return or disclosed by the report of any investigation of the income or return. The information shall be furnished to the Multistate Tax Commission, the federal or state officer or his or her representative, or the officials of Mexico for tax purposes only. Except when furnished pursuant to a written agreement, information furnished pursuant to this section shall be furnished only if the request is in the form of an affidavit under penalty of perjury stating that the purpose for the request relates to an investigation of the tax specified in the request and that the information will be used in the ordinary performance of the applicant’s official duties.

(b) Notwithstanding subdivision (a), and except as otherwise provided in 19551.1, tax officials of political subdivisions of this state shall request information from the Franchise Tax Board by affidavit only. At the time a tax official makes the request, he or she shall provide the affected person with a copy of the affidavit and, upon request, make the information obtained available to that person.

(c) For purposes of this section, “reciprocal agreement” means a formal agreement to exchange information between national taxing officials of Mexico and taxing authorities of the State Board of Equalization, the Franchise Tax Board, and the Employment Development Department. Furthermore, the reciprocal agreement shall be limited to the exchange of information that is essential for tax administration purposes only. Taxing authorities of the State of California shall be granted tax information only on California residents. Taxing authorities of Mexico shall be granted tax information only on Mexican nationals.

(d) This section shall become operative on January 1, 2019.