It means that the product can be plugged in and ready to use without any additional drivers.

So instead of having to use your computer to add additional audio in/out, you can just plug it up to your MPC it becomes another major feature of standalone.

Class compliance also applies to controllers.

So if the report is correct, you could add a controller like the Nektar P1/4/6

http://www.nektartech.com/panorama-p1.html )

which is one of the deepest midi DAW controllers on the market could be plugged up to give you deeper control with multiple faders, knobs and buttons.

then you could plug in audio interface to add a few more I/O.

You'd basically have a very similar setup to a PC daw in a standalone unit, albeit with a limit of 8 audio tracks and and no offline VST's.

It's a pretty dope feature. I know my Kronos can add controllers such as an MPD 32 via USB.

There may be limitations based on power consumption or older controllers that lack USB 3.0 capabilities with their firmware or whatever. To be able to do that with an audio interface is an extra level.