The galaxy isn't neatly divided into the Core Worlds and the Outer Rim. There are a bunch of other regions as well. Where do we draw the line when it comes to the regional knowledge skills?
The books are not very specific on this. I can think of a couple of ways to demarcate:
(1) Core Worlds covers everything out to and including the Expansion Region, Outer Rim covers everything farther out.
(2) Alternately, count everything outside the Core and Deep Core as "Outer"
(3) Here's the one that makes the most sense to me: Knowledge (Core Worlds) actually covers knowledge of the most "civilized" and politically powerful worlds; Outer Rim covers the less civilized and more powerless ones, wherever they are located on the map. It seems a bit odd to me that Knowledge (Outer Rim) would cover Malastare or Sluis Van, which I think of as urbanized planets rather than frontier outposts, or that Core Worlds would cover Byss and Tython, for example.