Sidechaining the multiband compressor

In the multiband compressor there is a graphic section where you can set the individual frequency ranges for the 4 different bands. Then in the sidechain section below there is a “Frequency” and “Q-factor” knob and I don’t know how this is different to the bands above. In the manual it says:

If the Side-Chain button is activated, this sets the frequency of the side-chain filter.

but I thought the individual bands above set this. Please can someone help me understand the differences.