NPOTB is the acryonym of "Not Part Of The Build".

An NPOTB change to source code may not need as stringent review as code that is constantly and automatically built by the Mozilla.

NPOTB changes don't affect the Tinderboxes for the main platforms. For example changes in files that are only used by OS/2 or BeOS don't interfere with building for other platforms, hence should have zero risk for breaking them.