Comentário oficial
The rekordbox button is to link the XDJ-RX2 to rekordbox in EXPORT mode, effectively using your computer's library as a USB device - all the work is being done by the RX2 itself.
The MIDI button is to use your XDJ-RX2 as a controller and audio interface for rekordbox in PERFORMANCE mode. In this mode, the software is doing all the work - and this would allow you to use the RX2 as a controller to playback audio files not supported by the hardware (eg. FLAC), as well as videos.