2008

produces a spill onto the object, Perimeter Alpha keeps all of the edges clean
because it relies on the scene alpha channel to derive it's effect.
Perimeter alpha highlights
Perimeter Applies highlight effect only to the perimeter of an object based
on Edge inferencing. Although not as precise as Perimeter Alpha, you might
need to use the Perimeter option at times when the alpha channel is
unavailable.
6692 | Chapter 20 Video Post