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Based on my understanding, if the owners of an OS want their OS to be officially called UNIX, they must get a certification from The Open Group.

macOS version 10.14 Mojave, as well as some (or maybe all, not sure) previous macOS versions have been certified as UNIX by The Open Group.

But why does Apple care if macOS is officially called UNIX?

closed as primarily opinion-based by Jeff Schaller, Rui F Ribeiro, Christopher, mosvy, Thomas Dickey Mar 4 at 20:49

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Apple "cares" as much as any other UNIX vendor. The Single UNIX Specification provides the framework to be qualified to use the UNIX trademark. The value of being branded as "UNIX" can be seen in The Open Group UNIX Certification Program which notes:

The Open Group UNIX standards offer the most stable, portable, and cost-effective applications development environment for a wide range of platforms from mobile devices to mainframes. For end-user enterprises, procuring certified UNIX systems ensures the highest level of availability, scalability, and maintainability for those who want to focus on their business with confidence in their IT.

It further states that the benefits of a UNIX branded system include:

  • Guaranteed consistency of services and behavior amongst UNIX® operating system implementations
  • Improved portability
  • Backward-compatibility
  • Faster development through the increased number of standard interfaces
  • More innovation is possible, due to the reduced time spent porting applications
  • An evolutionary approach that protects investment in existing systems, data and applications
  • The availability of UNIX systems from multiple suppliers gives users freedom of choice rather than being locked in to a single supplier
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Such certifications usually are costly, and it is why Apple can have the luxury to do that.

The certification process is usually required for ticking some boxes on the decision matrices/requirements for proposing solutions for (big) government/defense/health/corporate projects/contracts.

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