California Code, Streets and Highways Code - SHC § 6443

At least 15 days before each respective 15th day of April and October, until the assessment is paid in full, the treasurer shall mail, postage prepaid, to each owner of property described in the assessment, at his last known address, as appears upon the tax rolls current at the time of mailing, a postal card notifying him of the amount due and the date when payment is due from him on the assessment and stating that the payment is subject to penalty if not paid on or prior to the due date. The failure of the treasurer to mail the card, or the failure of the property owner to receive it, shall not affect the validity of any penalty or invalidate any act or proceeding.The card shall contain the following in at least 14-point boldface type:

If you do not pay this bill, at the request of the bondholder, your property will be sold by the treasurer.

This bill is not related in any way to your property tax bill. It must be paid separately.