Cal. Rev. & Tax. Code § 17952

For purposes of computing “taxable income of a nonresident or part-year resident” under paragraph (1) of subdivision (i) of Section 17041, income of nonresidents from stocks, bonds, notes, or other intangible personal property is not income from sources within this state unless the property has acquired a business situs in this state, except that if a nonresident buys or sells such property in this state or places orders with brokers in this state to buy or sell such property so regularly, systematically, and continuously as to constitute doing business in this state, the profit or gain derived from such activity is income from sources within this state irrespective of the situs of the property.