In New England there is a program for tuition ‘discounts’ from other NE state schools (only certain majors are eligible and its based on both which state you are coming from and which school you are attending). It lowers your tuition to something between IS and OOS. I know there are other similar ‘regional’ programs, so it could be one of the uses of the term you are coming across.
This tuition break is on top of any merit aid you may be getting (so it’s ‘stacked’). Unlike merit aid, it has no minimum GPA associated (except of course the school you are attending probably has a GPA policy regarding ‘adequate progress’). Also unlike merit aid, if you change majors to one that is not eligible, you lose the discount going forward. And yes, it only applies to tuition.