Form access modes provide a host of functionality. In fact, so much that not everything fits in the main UI. You can access all settings for form access modes on the Source tab. Here you can view, edit and add settings.
This means you can add things like required fields for a mode and validation rules.
PS, if you’re not seeing this UX, make sure you’ve upgraded to DRAPI 1.1, the source tab look and feel was updated in this version.