Gage Makers tolerances have been established by the American Gage Design Commitee and are referenced for building and calibrating all solid ring gages, plug gages, and fixed snap gages. Gage Maker tolerances are refered to as classes. For example Class ZZ tolerance is an open tolerance or economical tolerance. There are 6 classes of tolerance - class XXX, XX, X, Y, Z, and ZZ. See the link below for Gage Maker tolerances in inch and metric https://www.threadcheck.com/technical-documents/fixed-limit-gage-fact-sheet-pg28.pdf