I register that the Core S and DIO modules have designated ground connections in -X4: 3 and 4.
There is also a ground symbol above -X4:4.
This is also evident in the downloadable schematics on your website. Thanks!

However, the RevPi RO module does not have a ground symbol above -X4:4, and no available schematics (as far as I can see).
So the question is:
Should the RevPi RO module be grounded or not?
Thank you.
Best regards
Ruben Buchmann.