It's proprietary, apple-only, and probably patented so they can demand royalties from anyone else wanting to use it.
The author of FLAC has confirmed (in the Hydrogen Audio forums) that Apple is not using a variant of FLAC (with or without permission) despite it being under a royalty-free BSD license (the open source license even Microsoft says is not a cancer

).
The MPEG lossless standard is months away from being finalized and apple is apparently not using some variant of it either.
In the HA thread, compression was slightly worse than FLAC, performance was on par.
Sad that Apple is still up to their proprietary tricks just like Microsoft.