SEIN stands for Social Security Number for Employer Identification Number. Both the federal and California tax systems may ask for your SEIN when filing taxes. However, the purpose and use of the SEIN may differ between the two systems. The federal SEIN is used primarily for businesses and self-employed individuals to report income and pay employment taxes. The California SEIN is used to help determine residency status and potential eligibility for certain tax credits. It is essential to provide the correct SEIN when filing your taxes with both the federal and California tax authorities to ensure accurate processing and potential tax benefits.