List the workpiece material groups — the tree every grade's cutting data is keyed on.

Three levels: the ISO 513 application letter (P, M, K, N, S, H), a composition or structure subgroup beneath it, and a hardness-band leaf. A grade points at a leaf; when you register your own grade, pick the leaf that describes how it machines in the condition you buy it in — that choice, not the category alone, is what sets its cutting speeds and forces. Ordered by code, parents before children.