This is a legacy option: https://helpcenter.graphisoft.com/user-guide/65418/ says it works only with ARCHICAD 10 library.
It calculates with B (height), wall opening top oversize compatibility option, the subfloor thickness set for the marker, and the reveal sizes set with the built-in reveal UI page: https://gdl.graphisoft.com/tips-and-tricks/reveal-options-automatic-or-scripted
Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE