The masking margin is how many decibels a single affirmation sits below the music bed. Drag the margin control and the bed stays put while the voice level follows.
Masking has two sources: the bed above each line, and the voices themselves (every voice loops continuously, so they overlap and mask each other). More voices need less margin, but that help plateaus around four voices. A diverse cast (mixed genders, wide panning) stays separable, so it wants a slightly deeper margin.
The green band on the control is the suggested range for exactly your cast and bed; the gold tick is the suggested starting point. It is a range, not one right value. Tune within it by ear. It is a guide, not a guarantee, because the bed, speakers, headphones and playback volume all affect what you hear.