When it applies
The typical case is a taxpayer who moved, never received the correspondence, and discovered the problem when a refund was taken or a levy arrived. It also covers audits where documents were submitted and not considered, and assessments the IRS made from a substitute return it prepared itself.
The request needs to bring something new. Disagreeing with a conclusion the IRS already reached on evidence it already saw is not usually enough. Documentation that was never in front of the examiner is what moves it.

