The ES-6 is the counterpart to the ES-3 Lightpipe/CV Interface, offering up to 8 channels of CV input with output via an optical lightpipe connection.
The ES-6 itself has 6 input sockets; the remaining 2 channels are provided by an expansion module e.g. the ES-7.
Please note that a connected ES-3 is required for operation. The ES-6 gets its clock signals from the ES-3, which derives them from its lightpipe input.