Answer:
This check box activates, for example, the usage of the Simplex Method also for I-sections, which would otherwise be designed automatically using the Partial Internal Forces Method (PIFM), including redistribution.
If you deactivate the checkbox, the Simplex Method or Partial Internal Forces Method (depending on the selection) will only be used if a general cross-section is available (for example, from SHAPE‑THIN).
