Answer:
You can find this setting in the "General Data" of the model.
Here you can distinguish between the model types "3D", "2D - XY", "2D - XZ", and "2D - XY".
If you want to enter a slab as a 2D model and the axial forces within the slab should not be taken into account, set the model type to "2D - XY". If you want to enter a wall, set "2D - XZ".
Further information on this can be found in the RFEM manual, Chapter 12.2.1 General - Type of Model.