RE: What
The difference is merely a matter of paperwork. Before a vehicle has been type classified by the military (as Standard A, Standard B, substitute standard, etc.) they carry the prefix X. To slightly oversimplify, sometimes the paperwork for procurement moves faster than the paperwork for type classification, and hence hundreds of trucks of a given series have the X prefix. In this hobby, the XM211 and XM818 are the two most commonly found examples.
Take a XM818 apart, take a M818 apart, mix the pieces together, and you can build to trucks back from them without regard to where they originated.
Hope this helps,
David Doyle