It provides the files needed to begin Carbon development. The kit supports Mac OS 8.6 or later. Carbon APIs can be used to write Mac OS X applications that will also run on previous versions of Mac OS - from OS 8.1 or later.
Carbon is designed to provide a "gentle migration" path for developers moving from OS 9 to OS X. Carbon APIs are supported within OS 9, so developers can enhance applications for Mac OS X without ceasing support for customers using older operating systems.
The SDK is available for download to members of Apple's Developer Community (ADC).