Revenue Regulation No. 12-99, September 6, 1999. 3.1.2
Preliminary Assessment Notice (PAN). If after review and evaluation by the Assessment Division, or by the Commissioner or his duly authorized representative, as the case may be, it is determined that there exists sufficient basis to assess the taxpayer for any deficiency tax or taxes, the said Office shall issue to the taxpayer, at least by registered mail, a Preliminary Assessment Notice (PAN) for the proposed assessment, showing in detail, the facts and the law, rules and regulation, or jurisprudence on which the proposed assessment is based. If the taxpayer fails to respond within fifteen (15) days, from date of receipt of PAN, he shall be considered in default, in which case, a formal letter of demand and assessment notice shall be caused to be issued by the said Office, calling for payment for the taxpayer’s deficiency tax liability, inclusive of the applicable penalties.