Answer:
The cross-sections to be designed can be optimized as follows:
Select "Steel Design" and "Input Data" in the RFEM table window. Then, select the "Optimize" option in the "Use Other Section for Design" column of the "Sections" tab; see Image 01.
Now, you can perform the design again for this cross-section. Since the internal forces can change with new cross-sections and thus new stiffnesses, there is the option to export the optimized cross-sections to RFEM/RSTAB using the shortcut menu; see Image 02.
The described cross-section optimization only applies to standardized cross-sections. For further optimization, we recommend the Optimization & Costs add-on.