the USB device port (the rightmost port) is the port that can do USB audio streaming to your computer (only stereo at the moment, but will be updated to 8 channels as soon as I am finished with the update).
it cannot send/receive MIDI to/from your computer. In theory it can but there is no driver for that right now on the side of the SSP.
the USB host port in the middle is the one you use for your USB midi class compliant controllers. You can use them with the midi modules in the SSP.
The leftmost USB port is a high speed port, only for USB-ethernet adapters or other USB 2.0 high speed devices. It does not support full or low speed devices like USB MIDI controllers.