Every digital interface works with a digital clock, which sets the sample rate, e. g. 44.1 kHz. If two or more devices were digitally connected they need to run with the same clock, the need to be synced. Otherwise the audio will drift away sooner or later, resulting in crackles, distortion and/or different speed.

The Babyface can run as clock master or slave.
Master: internal clock (= the Babyface uses the sample rate set in your software).
Slave: If an external interface is digitally connected to the optical input of the Babyface it can receive the external word clock via the optical input and run as clock slave to this master. Example: Connected to another interface (e. g. Fireface UFX) the Baybface uses the sample rate set in this interface.

It's not possible to connect another device digitally to the Babyface, if it does not provide an ADAT/SPDIF I/O like a Duet. You can connect any hardware with a digital SPDIF or ADAT I/O to the Baybface. This could be a 2- or 8-channel AD/DA converter, a synthesizer, a digital mixer, a hardware effect unit or even a CD player with optical output.

best regards
Knut