Lots of HR info in this article if you’ve not seen in
https://thesufferfest.com/blogs/training-resources/heart-rate-and-training-everything-you-ever-wanted-to-know-but-were-afraid-to-ask
Although there’s also a discrepancy there versus the zones that the app actually uses! - 4b in the article is start of 5 in the app. However once you’re above “threshold” HR is not so useful as a target (still really useful for analysis but that’s for elsewhere) so anything above zone 4 could just be called the suffer zone.
(I’ve changed the category to training as the SUF coaches may have some more informed info to add relating to HR and zones)