Means 90 degrees +/- 0.00 (in theory)
Means as close to 90 as you can possibly get it (in practice)
Remember the 3,4,5, rule in a triangle will produce an exact 90 where the 3 & 4 leg meet.
I have never heard of someone loosing their nuts over it but who knows.

GM Automotive Manufacturing Engineer for 30 years desigining, purchasing, and installing final assembly lines for the FWD Halfshafts for GM and even Toyota & Chrysler worldwide. Something new every day around the world. Had enough and retired 3 years ago when I was constantly asked to support an...