The World of Avlis

I used a character leveler to generate an epic monk and went into combat.
I couldnt see any indication one way or another whether they stack or dont stack.

Any big brained code warriors out there know the answer?

-aka Shen, the monk that wants to hide a lot

Concealment never stacks and the maximum a character can get by any means is 50%, equivalent to full concealment. Invisibility, improved invisibility, self concealment V and empty body (and in Avlis, blur until it's fixed) will give you a 50%.

EDIT: With the toolset creatures with higher concealment ratings can be made but this is not available for PCs.