RMSC - stands for RingModulationSideChain

It's a procedure where the the Signal gets reduced by the exact Amplitude Value of the Sidechain Signal. This provides that the sum of both signals will never exceed the maximum Amplitude and won't start clipping.
It's typically used in addition to Sidechain Compression/ Volume Shaping to gain some more headroom in the mix.

• Stereo Linking for independent or linked processing

• Hold: Holds the maximum value of the ducking curve for a specific amount of time

• Release: Logarithmic smoothing of the ducking curve

• Dry/Wet: Depth control of the ducking effect

• Scope for visual feedback with variable buffer size

• Backwards compatible with older Ableton Live Versions / Following Ableton color themes

You can find an in depth tutorial on the topic of RingModSidechain on my YouTube Channel along with some other tutorials.