That's the length of the BB spindle. You need this to correctly match your crankset for proper chainline.

So, choosing a BB is two parts. The threading (and width) of the BB cartridge needs to match that of your frame's shell. Then the spindle length and type of interface must be compatible with your crankset.

If you're replacing a worn BB, measure it (or look for the markings) to determine the correct spindle length.