XPRI is filled when you generate previs. I don't think it is actually used in-game, though I could be wrong
Just the other day, I made a patch for Greekrage's Fort Hagen and Boston Natural Surroundings.
Quite unexplicable at first, the landscape at the entrance gate would be heavily deformed in-game, whenever I loaded the patch. The deformation could neither be seen when just loading BNS or just loading the Fort Hagen Mod. There was no evidence of it in Creation Kit while making/editing the patch either.
After deleting a bunch of XPRI records from the patch, the landscape was fine.