Estimated tax bill
A bill a town may send, by resolution, when the county tax board hasn't certified all the tax rates by the mailing date. It uses an estimated levy, and a later reconciled bill corrects the amount once final rates are set.
New Jersey example
Say the county hasn't certified rates by the June 14 mailing date. Your town can send an estimated August bill using levies generally set within 5% above or below last year's. When final rates arrive, your November bill is the full year's tax minus what you already paid. If a bill is mailed late, you get at least 25 days after mailing to pay before interest.
Official source
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.