Core Foundation makes it possible for the different frameworks and libraries on Mac OS X to share code and data. Carbon, Cocoa, and Mac OS 8 and Mac OS 9 applications, libraries, and frameworks can define C routines that incorporate Core Foundation types in their external interfaces; they can thus communicate data--as Core Foundation objects--to each other through these interfaces. Indeed a Carbon developer could, for example, provide a Core Foundation-based service that can be used by any running application on a Mac OS X system regardless of its origin.
Core Foundation also provides "toll-free bridging" between certain services and the Cocoa's Foundation framework. Toll-free bridging enables you to substitute Cocoa objects for Core Foundation objects in function parameters and vice versa.