Sidechain/Ducking Source : specifies different audio source (if any) which will be used as side-chain audio track (helper track, smother track) to adjust current audio.
Compression value applied to the current audio source but values to trigger this compression are taken from side-chain audio source (helper track, mute track, smother track). So, if side-chain track audio fits threshold gate then the main track is compressed (applies Ratio ... Output Gain).