The IRC and the CSSB differ somewhat in their recommended installation requirements. Local authorities having jurisdiction (AHJs) may also have differing requirements. The CSSB-97 Grading Rules have been adopted by the IRC, the International Building Code (IBC), and the National Building Code of Canada (NBC).
Shakes are available in four grades: premium, #1, #2 and #3.
Wood shingles are graded according to how they’re cut and the location of their knots and checks. You probably won’t be able to identify the grade, and you’re not required to.
Manufacturers’ warranties vary and may require installation by a certified installer. A common warranty caveat that is widely accepted is that installation must be performed by a CSSB-approved installer.