The ultimate Description forthcoming. These live in /System/Library/PrivateFrameworks. Use them at your own risk, because Apple will feel free to pull the rug completely out from under your feet.
Private frameworks are a frequent topic on UndocumentedGoodness pages
If you use a PrivateFramework, such as CALCore, in your program, is that legal? Yes. And second, if Apple removes that framework from OS, is it illegal to bundle it in your application? Heck no! This isn’t that different from distributing anything of Apple’s, like say, OS 10.3.
There’s no obvious legal reason why you wouldn’t be able to link to any library on a user’s system. Distributing somebody else’s software, however, is always illegal unless he gives you permission. Of course, I’m a coder, not a lawyer, Jim.