[CentOS] how is "binary compatibility" determined?

Hi all!

We've seen quite a few references on the list to "100% binary compatibility with upstream". What I am curious about is, how precisely is this determined? All the ways I can think of for comparing how two systems might work seem flawed in some way (e.g., using some sort of checksum; unit testing; verifying build parameters). I did some searches both at centos.org and google, but couldn't find anything specific about the test(s) used to determine compatibility.

I have no particular reason for asking, other than wanting to be able to explain it to others if someone asks me.

--keith