I don't really know how to explain it without writing an entire book on it.
EDIT: Look at Alexander Blade's post, explains them a bit better. But in short, XSC is V's SCO.
The main difference between SCO and #SC is:
SCO is simple binary file and #SC is RAGE resource like models, collisions etc.
Which, the 'X' in XSC is dependent upon the platform the script was made for:
CSC - Cell
XSC - Xenon
WSC - Windows/PC
OSC - Orbis
DSC - Durango
Of course, those last 3 are unconfirmed, but are most likely what they are going to be seeing as they have a few natives relating to those platforms.
If you look carefully into V's code you will see all except "W" platform, instead here will be "Y" platform which I guess Windows/PC x64.
I doubt there'll even be easy ways to handle 64-bit pointers in game resources from a 32-bit game executable except for replacing all pointers with some weird class (unless that's exactly what happens in V pgStreamable pointer relocation?).
Not sure about "W", maybe we will have only "Y"
They already uses 64-bit pointers in PSO resources, such as .#TYP, .#MAP etc.