Thanks Gerry,
I've went at looked at the DRO & OEMDRO pages of the Wiki.
I see DROs don't have a number but an FCODE (still to learn what fcode means). For example "X DRO" has fcode "0"
I can also find "X DRO" in the list of OEMDROs in the Wiki, and there it has OEMCODE 800.
Using Screen4 I double clicked on the X DRO and it came up as "System Function" "X Position". I'd hazard a guess "fcode" means system function and X Position is system function "0"
So if I was designing a screen with Screen4 and I wanted to create the basic X DRO, I'm guessing I could use either "System Function" "X Position" or "OEM Code Function 800", and I'd end up with the same result.
Keith.