In a nutshell

The optional CL-6 expander allows you to add to the 664 mixer six faders and a 4 channel LED VU meter, REC and STOP buttons, to use the 6 direct-out connectors of the 664 as line inputs and to add 6 tracks to the recorder taking it up to 16 tracks.

This rate applies if taken alone without Sound Devices 664 Field Mixer.