The Sidechain Limiter is a Multiband Limiter with an additional input - the so called sidechain. This input is treated like a single band of the limiter which means that its signal is taken into account for the multiband processing. The sidechain is also mixed into the main signal afterwards.
Use Sidechain Limiter for mastering a multitrack project where some of your (important) single tracks perish against the overall mix.
E.g. a lead vocal track is sometimes superimposed by other instruments:
Route an AUX send from the vocal track to the sidechain. The send level
of the AUX determines the presence of the lead vocals.
Please refer to the Multiband Limiter page for more details.