Ordered a gear vendor, help with deciding gears

Personally, I never liked the gear vendor unit hanging off the back of the trans like that. I see that they have a shorty model now, but if imma do that, I might as well run a 518 and be done.

That said, how's come I can't split gears with the OD unit on a 518? I don't want to, but I'm not a transmission guy and I'm just curious.