That measurement(The proper way to do it) is the height with respect to the rear axle height. +0 would be 13" on a perfectly 26" tire, +9.6mm would be taller, -15mm would be lower ect.while were on this subject, im looking at a nuke mega tr frame.. it says the bb is +0.
where does that stand in today's mtb world? im looking for a decent all around frame.
edit:
im also considering the AM (more aggressive version) that has a BB of +9.6.