I know how you feel... everytime I start gathering info on this, I get conflicting info, and conflicting terms being used... Doh!
As Q has so eloquently explained is how I was understanding the whole scenario...
As in my case also, eventually, I plan on switching my gearing from 2:56 (posi) to (again, from what I've been told) a 3:08 or 3:23 (Posi) preferably and I should be able to use my carrier (the part that's installed into the housing) to put the new gears (ring and Pinion that bolts into the carrier) into.
From some of the info (nothing difinitive yet?) I should be able to go up to at least a 3:08 ring and pinion, and as Q mentions possibly a 3:23 before having to switch carriers if I wanted to go to a better gear combo than that.
So this tread is a great one to help spell things out a little better!