Hi Robst,
The funcionality of Pin 20 depends on the cFP module that you are using. You will need to check the Operating Instructions for the terminal assignments. That's the only way to know if you need the pin or not. It may be required for only 1 channel. In that case, you won't be able to use that particular channel.
Hope this helps.
Saludos,
Erin
NI Mexico