Answer:
In RF‑/STEEL EC3, the uniaxial bending stress is always related to the local y-axis of a member and the local y‑axis of a member is always assumed as the major axis in the case of symmetric cross-sections; therefore, it is only possible in such a case to change the cross-section description from "100/5" to "5/100"; see the image. Thus, the stability analysis is performed accordingly.
