The masked application is for when you have two different light sources, each illuminating an area.
Two instances, one with CAT, the other without, is different: the 2nd instance is basically a channel mixer. There’s even a preset for that.
@BorisRabit : you’ll probably like the new rgb primaries module coming in darktable 4.6, which is an alternative interface to the channel mixer, making it easier to use. It will even be embedded into sigmoid (but can also be used on its own). New Primaries module