A lumber grade is best understood as a certified minimum: boards are sorted into bins, and everything in the No. 2 bin is guaranteed to meet or exceed the No. 2 design values for its species. The code’s span tables then design to that floor — the IRC deck-joist table assumes No. 2 grade, the header table is built on No. 2 minimum properties, and the rafter tables encode the No. 2 rows because that is the grade lumberyards actually stock. The tables accept "No. 2 or better," so a Select Structural board in a No. 2 application is simply quiet overstrength, while stud and utility grades fall below the floor and are not covered at all.
How much the bin matters depends on which property you ask. Premium grades run 50 to 65% stronger in bending than the workhorse No. 2 — spruce-pine-fir jumps from 6.0 MPa to 9.0 at Select Structural, southern pine from 7.6 to 11.7 — but stiffness moves far less (SPF’s modulus of elasticity is listed identically at both grades). Since long spans are often governed by deflection rather than strength, paying for a fancier grade buys less extra span than the bending numbers suggest, and prescriptive tables never reward overstrength anyway. Grade always travels with a species group — Douglas fir-larch, hem-fir, southern pine, spruce-pine-fir — matched to the stamp, never to the wood’s color or weight; Europe compresses the whole pair into one EN 338 strength class, where any board certified C24 simply designs as C24.