Additional decks are provided through the decks tab. The differences between the main decks(A and B) and the decks on this tab are:

To load one of these decks with track either drag a track from the playlist onto the Decks tab then drop onto a deck or, click the deck button on the playlist. Also, you can use the main menu or the playlist right click menu and select the appropriate deck. Hotkeys are also available to quickly load the selected track.

These additional decks also come with MIDI controller support. Just associate the specific deck control with the appropriate MIDI command and you will be able to control the additional decks from your controller.