Nits

A nit is one candela per square metre (cd/m²), a unit of luminance. It describes how much visible light a surface emits in a given direction. Unlike a brightness percentage, it is an absolute unit. Apple’s video glossary

BetterDisplay can show nits using device information and configured brightness ranges. A displayed value may be calculated from those ranges; it should not be treated as a new reading from a measurement instrument. Likewise, 50% on two displays need not produce the same luminance.

The limits also describe different things: SDR maximum, HDR peak and the range available for upscaling are not interchangeable. Changing a configured range changes BetterDisplay's mapping, not the panel's physical output capability.

See Brightness and HDR for those relationships. General Settings explains the range settings.