They are exactly what they imply.  PPC are cels 
made after the anime is done-- so take something 
like Get Backers or Samurai Deeper Kyo.  Both of 
these were CG shows.  But there are PPC 
available for them.  These are unique and as far 
as I know one of a kind cels that are made after 
the show is done.  I guess they do this for us 
cel geeks that drool over how many hot bishis 
there are in anime that we'll never have because 
they're all CG...  Now if only Angel Sanctuary 
had some PPCs done.........

Rekka