Archived - What's a "computer-specific Mac OS X release"?
What does "computer-specific Mac OS X release" mean?
Most new Macs come with a "computer-specific" release, or build, of Mac OS X. In fact, there's a great list in this document.
Computer-specific versions of Mac OS X include special software that lets Mac OS X work seamlessly with the new model's hardware, and is required for the computer to work correctly. That's why you usually can't use an earlier version of Mac OS X than the one that came with the computer. The special software is not included in Mac OS X reference releases or software updates that were released before the model was introduced.
More about builds, versions, and reference releases
Computer-specific versions of Mac OS X can have the same version number as an existing Mac OS X reference release or software update, such as "Mac OS X v10.3.3", but a computer-specific version of Mac OS X will have a different build number, such as "7G43". Again, a handy list of build numbers is right here.
Important: Reference releases and software updates that have similar version numbers, such as "Mac OS X v10.3.3", but earlier build numbers, are not designed to work with your new computer.
Tip: You can easily tell which build number is earlier by breaking it down: If the first number is the same, look at the letter. If they are the same, look at the last two numbers. The higher the number, the more recent the build. "7G33" is earlier than "7G43" (because 33 is less than 43), and "6F25" would be earlier than both (because 6 is less than 7).