California Code, Revenue and Taxation Code - RTC § 25127

(a) Patent and copyright royalties are allocable to this state:

(b) A patent is utilized in a state to the extent that it is employed in production, fabrication, manufacturing, or other processing in the state or to the extent that a patented product is produced in the state. If the basis of receipts from patent royalties does not permit allocation to states or if the accounting procedures do not reflect states of utilization, the patent is utilized in the state in which the taxpayer's commercial domicile is located.

(c) A copyright is utilized in a state to the extent that printing or other publication originates in the state. If the basis of receipts from copyright royalties does not permit allocation to states or if the accounting procedures do not reflect states of utilization, the copyright is utilized in the state in which the taxpayer's commercial domicile is located.