California Code, Revenue and Taxation Code - RTC § 25101

When the income of a taxpayer subject to the tax imposed under this part is derived from or attributable to sources both within and without the state the tax shall be measured by the net income derived from or attributable to sources within this state in accordance with the provisions of Article 2 (commencing with Section 25120). However, any method of apportionment shall take into account as income derived from or attributable to sources without the state, income derived from or attributable to transportation by sea or air without the state, whether or not the transportation is located in or subject to the jurisdiction of any other state, the United States or any foreign country.

If the Franchise Tax Board reapportions net income upon its examination of any return, it shall, upon the written request of the taxpayer, disclose to it the basis upon which its reapportionment has been made.