Cal. Rev. & Tax. Code § 19533

(a) In the event the debtor has more than one debt being collected by the Franchise Tax Board and the amount collected by the Franchise Tax Board is insufficient to satisfy the total amount owing, the amount collected shall be applied in the following priority:
1. (1) Payment of any taxes, additions to tax, penalties, interest, fees, or other amounts due and payable under Part 7.5 (commencing with Section 13201), Part 10 (commencing with Section 17001), Part 11 (commencing with Section 23001), Part 32 (commencing with Section 61000), or this part, amounts authorized to be collected under Section 19722 of this code, or payment of advanced premium subsides in excess of the amount allowed under Title 25 (commencing with Section 100800) of the Government Code.
2. (2) Payment of delinquencies collected under Section 10878.
3. (3) Payment of any amounts due that are referred for collection under Article 5.5 (commencing with Section 19280) of Chapter 5.
4. (4) Payment of any delinquencies referred for collection under Article 7 (commencing with Section 19291) of Chapter 5.
(b) Notwithstanding the payment priority established by this section, voluntary payments designated by the taxpayer as payment for a personal income tax liability or as a payment on amounts authorized to be collected under Section 19722, shall not be applied pursuant to this priority, but shall instead be applied as designated.