Yes you’re correct about having to do with the tolerances. If you’re talking about the ENCO Class ZZ Steel Pin Gage Sets then the minus tolerance set would mean that the pin ‘should not’ exceed the stated diameter but could fall under the stated size by -0.0002. So as an example a pin that is say .030 dia. would not exceed .030 but could fall below .0002 to .0298. The Plus Tolerance sets would be just the opposite. In many machine shops and most tool rooms you’ll find both sets on hand but given the choice of just one set I prefer the minus tolerance set. Just my personal preference though.
Hope this helps Tom,