What does this square box next to the fill effect mean?

[Elsa]

 it looks like a matte, but l can't figure it out by myself. Please help

[Roland Kahlenberg]

That Square Box is a visual indicator that the effect's Compositing Options has been enabled. Compositing Options are enabled to link an effect to one or more masks on the same layer as the effect.

More info here - https://helpx.adobe.com/after-effects/using/mask-reference.html

[Elsa]

Thanks a lot!