Compatibility
You can determine what features are available on a Macintosh computer using
various attributes, versions, and features of particular software and hardware
available on the currently running system.
ensure that your application is compatible with previous versions of
Macintosh system software as well as with new releases of Macintosh system
software.
These guidelines can help you develop your application so that it is compatible
across the Macintosh family of computers. The guidelines also provide
information on how to make your application compatible with A/UX and how to
design your application so that it can be easily localized for use in other
regions.