The issuance of a valid formal assessment is a substantive prerequisite to tax collection. A formal assessment contains not only a computation of tax liabilities but also a demand for payment within a prescribed period, thereby signaling the time when penalties and interests begin to accrue against the taxpayer and enabling the latter to determine his remedies therefor. Due process requires that it must be served on and received by the taxpayer. (Commissioner Of Internal Revenue vs. South Entertainment Gallery, Inc., G.R. No. 223767, April 24, 2023)