An LOA is the authority given to the appropriate revenue officer assigned to perform assessment functions. It empowers or enables said revenue officer to examine the books of account and other accounting records of a taxpayer for the purpose of collecting the correct amount of tax. An LOA is premised on the fact that the examination of a taxpayer who has already filed his tax returns is a power that statutorily belongs only to the CIR himself or his duly authorized representatives. (Medicard Philippines vs. Cir, G.R. No. 222743, April 05, 2017)