Bottom and top plate sizes come from the tables for the storey and roof type (with reduced plates allowed where studs sit over joists, blocking or slab, or where plates resist no uplift), combined plates rated by averaging as the clause shows, and roof beams, struts, girder trusses and similar supported directly by jamb studs, concentrated-load studs or posts; non-loadbearing walls may use the minimum sizes table in any grade.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.