"Mixable Track" means it is 32-count phrased and is mixable with other similar tracks to create a smooth, seamless, non-stop transition between tracks - much like a DJ would to. The beginning and end of songs are seamlessly beat-matched for a smooth and uninterrupted transition.
"Fade Track or Radio Version" means the tracks are not 32-count phrased. They are typically shorter and are usually exact copies of the version of a song you would hear on the radio. These tracks start without a long intro and simply fade out at the end or just end abruptly. These are ideal for classes that do not require a non-stop mix or are also great to put at the end of a mix as a cool down or ending track.
This is what a Mixable Track looks AND sounds like, going from one track to another.
And here is what a Fade / Radio Version track looks AND sounds like: