https://en.wikipedia.org/w/index.php?title=Red_Hat_Enterprise_Linux#Extended_Update_Support_(EUS)_/_Z_Tree I do see that EUS source packages are available to customers in the errata portal, but I was looking at the article for Red Hat Enterprise Linux on Wikipedia, and saw a note there to the effect that rebuilds don't publish updates from EUS because Red Hat doesn't publish it for rebuilding. I vaguely recall that CentOS pulled source from an FTP source that the claim might describe, but I can't find any information that I think is really conclusive. The question is mostly trivia, but if the claim on Wikipedia isn't accurate, then I'll update that page.