For load combination, RSTAB and RFEM automatically perform the second-order analysis. Which criteria are used here? Can I deactivate this?
In the result combinations, the results of the load cases and load combinations are just "brought together." You get an envelope of the internal forces (max and min values).
For load cases and load combinations, you have the option to specify the method of analysis in the "Calculation Parameters" dialog box (menu "Calculation"). You can calculate load cases and combinations by three methods separately. Because in result combinations the results of load cases and/or load combinations are evaluated, the result combinations can therefore contain results according to statically linear, second-order, or large deformation analysis.
For load cases and load combinations, the results are always unambiguous internal forces. For more information, see chapter 5.5 of the RSTAB or RFEM manual.