Stud sizes come from the Supplement tables for common studs (notched or not, single or upper storey, lower storey), studs under concentrated loads (using area supported, half the ceiling area for hanging beam loads), jamb studs (increased for concentrated loads on lintels by location, with rules for full-length and secondary jamb studs), internal loadbearing studs, gable and skillion end studs and mullions; for doors up to 900 mm, jamb studs may equal common studs when stiffened and not carrying concentrated loads.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.