I'm not sure, but I will hazard a guess.
CDEF Barbarian to powerful MIGHT be a constant value that helps the AI for barbarians determine whether to accept certain offers. I've noticed that barbarians are smart enough to surrender when you have a large force nearby, so maybe that's what that does.
#_CDEF_BARBARIAN_DISPLACEMENTDIFF_ MIGHT be a constant value that regulates how likely it is that letting barbarians settle on your land will change the province culture to their culture.
Like I said, I'm not sure, so I could be wrong. My ideas shouldn't be that hard to check out, though.