Until an official answers you, i can share my personal findings.
When using precise i set detection to 80% and auto-mark to 95% I've had "instrumental" and "normal" versions of tracks show as 89% similar for example. Well, the music is of course the same, but i don't want to remove either. Same for live and album tracks. It probably depend on your collection but i wouldn't trust anything below 90% for auto-mark.
