According to Revenue and Taxation Code §5362, the Assessor of the county in which an aircraft is habitually located shall assess the aircraft at market value. If you received this form in the mail, the Assessor's records indicate you are the owner of the aircraft identified on page 1 of the form.
The form available for download is to be used only as an attachment to the original BOE-577 mailed by the Assessor (Revenue and Taxation Code §441.5).