By default, a field is set as optional, giving your patients the choice to provide an answer, or not. If you need a response for a particular question, go to that specific field and choose required to turn it to a mandatory field.
If you want to switch it back to optional, simply choose optional again.
What this means, for screens
Screens without any required fields show a skip button. If at least one field is required on a screen, no skip button will be shown.