The ADAT optical digital interface is common on many digital audio systems, even if it is less well-used than perhaps it deserves to be. Each optical port allows up to eight channels of data to be transferred via a simple, low-cost light-pipe, making it ideal for carrying multiple channels between pieces of digital audio equipment - such as connecting an external set of mic pres like the Clarett OctoPre to a computer audio interface. Most of the larger (and some of the smaller) Focusrite interfaces include ADAT input or I/O and as a result it's easy to expand the number of inputs (and often outputs) available to your interface using this method.
ADAT - The history
The "ADAT Optical Interface" format (these days, we tend to call it simply "ADAT interface", "ADAT Optical" or "ADAT I/O") was developed by audio manufacturer Alesis for connecting their videotape-based digital multitracks ("ADAT" stands for "Alesis Digital Audio Tape"), and although the machines it was designed for have long since gone, the interface format, conveniently carrying up to eight digital channels, is still with us - it's widespread and well-known, and many devices include it.
The ADAT Optical format uses standard, low cost optical "TOSLink"light-pipes, originally designed to carry S/PDIF digital stereo signals between home hi-fi components. But instead of the two-channel stereo of S/PDIF (with which it is not compatible), at traditional standard sample rates (44.1/48kHz), ADAT Optical carries up to eight channels on a single light-pipe. As the need for higher sample rates became more prevalent, a system called S/MUX (Sample Multiplexing) was developed, enabling first four channels to be carried on a single lightpipe at 88.2/96kHz sampling, and then (with the advent of S/MUX 2), two channels at 176.4/192kHz.