Cal. Rev. & Tax. Code § 19441

(a) The Franchise Tax Board or any person authorized in writing by the Franchise Tax Board is authorized to enter into an agreement in writing with any person (or the person or estate for whom that person acts) in respect of any tax, interest, penalty, or addition to tax levied under Part 10 (commencing with Section 17001), Part 11 (commencing with Section 23001), or this part for any taxable period.
(b) If the agreement is approved by the Franchise Tax Board, itself, within the time as may be stated in the agreement, or later agreed to, the agreement shall be final and conclusive, and except upon a showing of fraud or malfeasance, or misrepresentation of a material fact:
1. (1) The case shall not be reopened as to the matters agreed upon or the agreement modified, by any officer, employee, or agent of the state, and
2. (2) In any suit, action, or proceeding, the agreement, or any determination, assessment, collection, payment, abatement, refund, or credit made in accordance therewith, shall not be annulled, modified, set aside, or disregarded.