Applies To: Focusrite Control, Scarlett & Saffire Mix Control
Internal: If you are using your interface by itself without any external gear, select "Internal" as the Clock Source as this means the unit will sync to itself. If you are using external gear with your interface, BUT, you are sending clock signal to that device (or multiple devices), your interface will need to be on Internal.
S/PDIF: If you are using your interface with a secondary stereo sound source which does not have a selectable sync/clock setting of its own, select S/PDIF as the Clock Source and your unit will sync to the secondary piece of gear.
ADAT: If you are using your interface with another multiple-input device and want all inputs to show individually, you would connect them via ADAT. Most ADAT units have the option to choose Clock Source and it is not imperative as to which unit is the host. So long as one unit is set to Internal to be the Sync Host and the secondary unit is set to ADAT to follow the sync information.
Wordclock: Also known as BNC, Wordclock will not host your audio but will host the the most reliable form of sync between units. Wordclock is mostly used as the Clock Source when a digital 'master' clock is introduced as it can be used to chain multiple units together in sync regardless of audio signal flow.