"Dry" is an industry term that means "Dry Contacts", which means it is a "Relay contact",
i.e. NOT a Sinking or Sourcing contact.
So inside the machine, there are small relays that close (or open if it is NC).
It also means you can pass through your interface voltage (5vdc, 24vdc etc.) without worrying (unless your out side the relay ratings).
Look in your manual.
But for the most part almost any relay will handle 5v to 110v sometime more, it could be
AC or DC as well. Dry contacts give the user the greatest interfacing options on control systems.
Scott