I don't think there are hard and fast number for that. Personally, I would consider idle-1800 low rpm, 1800-3800 mid range, 3800 - red line as high rpm.

Other may adjust those values a few hundred rpm either way.

Ask me again next week and I may adjust those number too!!

FYI - I run the CALMINI cam and I would say usable RPMs are between idle-5000, the power really flattens out after 5000. It idles pretty strong down to around 550-600.