Apparently it was for legal reasons. Apple was sued by the Open Group for describing Mac OS X as Unix® so they decided it was cheaper to bring it up to spec rather than to fight the lawsuit.

https://retrocomputing.stackexchange.com/questions/27095/why...

https://www.quora.com/What-goes-into-making-an-OS-to-be-Unix...

This is incorrect.

More than 1 Linux distributions have been tested, passed, and listed in the past... But you need to pay again every year, and recertify every new version, and they didn't bother keeping it up.

I described them and what this meant in some depth in this 2023 article:

https://www.theregister.com/software/2023/01/17/unix-is-dead...

Enterpise espeically banking need standard and certification for reliability thats why , also certain Linux distro had certified before from China based on RHEL.

This is UNIX 03 certified OS with OSS alternatives from I gathered on internet :

- Apple OS X 10.7 - Apple macOS 26 , provide OSS Darwin 9 - Darwin 26

- Oracle Solaris 10 / 11 , provide OSS OpenSolais , then Illumos

- EulerOS 2.0 , based on RedHat Enterprise Linux / RHEL 7 and 8

- Inspur K-UX 2.0 , based on RedHat Enterprise Linux / RHEL 5

- Inspur K-UX 3.0 , based on RedHat Enterprise Linux / RHEL 7

It is specific Linux distributions that would be Unix-certified. Huawei had one; not sure if it has ever been on this page.

Enterprise. Checklists.

It was a legal problem, and an interesting story: https://www.quora.com/What-goes-into-making-an-OS-to-be-Unix...

And Gnu’s Not UNIX.

Linux can’t be listed, because Linux is not an operating system. It’s just a kernel. Unix specifies a whole operating system, which includes things like libc, cat, sh etc.

At best a distro could be certified. But that certification would not automatically extend to other distros.