What does the -55 boundary rule signify with respect to NMD?

Question

Quizplus · Accepted Answer

The -55 boundary rule states that only stop codons at least 55 nt upstream of the final intron trigger NMD. This means that if a premature stop codon (PTC) is located more than 55 nt upstream of the final intron, it will trigger NMD and lead to mRNA degradation. However, if the PTC is located within 55 nt of the final intron, it may be bypassed by NMD and still lead to the production of a truncated protein.

What Does the -55 Boundary Rule Signify with Respect to NMD