National availability means how many of that option will built nationwide. Then you have to hope your dealer gets an allocation for them. The national quantity might be 200 and your dealer not get any of them.
That said, watching the allocation may give an idea of the direction they are going with constraints. After watching it for a couple of months, seems like they go from 0 to like 30 to like 100 to like 200 some and then the constraint is lifted. Once it starts moving that that direction, it seem like it takes a month or two before it’s lifted. While it’s in the 30-200 range, it’s almost like winning the lottery to have your order selected.