[CentOS] Differences from upstream RHEL

At work, we use some commercial software, that names RHEL6 as a supported OS, but not Centos6. I would like to know the difference between Centos and RHEL, in order to claim (or not) that we can support our users on Centos instead of RHEL.

I see the release notes, that say "Packages modified by CentOS," but it's not clear what the modifications are. I have been browsing around for these details, and have not yet found specifics of *what* was modified in those packages.

Can anyone please direct me toward details of what's modified in the packages that centos modifies?