Cal. RTC § 19058

(a) If the taxpayer omits from gross income an amount properly includable therein which is in excess of 25 percent of the amount of gross income stated in the return, a notice of a proposed deficiency assessment may be mailed to the taxpayer within six years after the return was filed. Additionally, in the case of a corporation, a proceeding in court for the collection of the tax may be commenced without assessment at any time within six years after the return was filed.

(b) For purposes of this section, all of the following shall apply:

(1) In the case of a trade or business, the term “gross income” means the total of the amounts received or accrued from the sale of goods or services (if the amounts are required to be shown on the return) prior to diminution by the cost of the sales or service.

(2) An understatement of gross income by reason of an overstatement of unrecovered cost or other basis is an omission from gross income.

(3) In determining the amount omitted from gross income, other than in the case of an overstatement of unrecovered cost or other basis, there shall not be taken into account any amount which is omitted from gross income stated in the return if the amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the Franchise Tax Board of the nature and amount of the item.