Letter Notice (LN) is considered notice of audit or investigation as far as amendments of Returns. Once taxpayer received an LN, taxpayer can no longer amend tax returns in relation to the years covered by the LN.
The guidelines provided under RMC 40-2003 in the issuance of LN has been upheld in the CTA Case No. 7093.
References:
Big AA Corporation vs. CIR (CTA Case No. 7093 dated Feb. 22, 2006)
RMO 13-2012
RMC 40-2003
Bustamante Lozada and Associates 