From the Dev Chat on 6/27/2008:
Can you break down the exact differences between PE and LP in the new engine? More specifically, certain guards with excellent BH but poor PE can score with some efficiency in the current engine with the dribble drive. Will guards now need LP to finish in the lane on a drive, or will LP only apply to post up's, tips, alley-oops, etc?(narcotico - Hall of Famer - 12:29 PM)
It's best to think of Perimeter as "outside scoring" and Low Post as "inside scoring". I know the term "Low Post" isn't the best descriptor for what that rating will mean. Scoring, both inside and outside, is impacted by Ball Handling, Speed, and Athleticism.