Real Estate Law
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My understanding of that document is that it indicates you may have some tax liability for the transaction. This is from the internal revenue service website which is the taxing authority of the US. https://www.irs.gov/Businesses/Withholding-of-Tax-on-Dispositions-of-U.S.-Real-Property-Interests
Actually, it says a buyer was to withhold 10% for tax purposes under a particular statute Foreign Investment in Real Property Tax Act. You may want to contact a tax attorney or a certified public accountant in California who could help you deal with this situation.