Cal. Rev. & Tax. Code § 19526

The Franchise Tax Board shall develop and maintain a taxpayer cross-reference file which shall be used as a part of a nonwage earner filing enforcement program. The taxpayer cross-reference file may include information from taxpayer identification information available from tax returns, application forms, other documents, and any other files as already exist and are maintained by the state’s major revenue agencies. The taxpayer cross-reference file shall only be used for the purposes of tax law enforcement and administration.