Sources: CD/SACD/Blu-ray/DACs/Music Streamers & Servers etc

 A "source" is where music begins in the system and is then feed to an integrated or pre-amplifier, so typicaly includes CD players, Blu-ray players, DACs, Tuners, Music Servers and so on. These particular components are often referred to as digital sources. 

Another typical source could also be a turntable but we have treated these seperately as an analogue source.