Mask settings.

The mask can be deactivated by the checkbox in the header.

UV Layer
Allows you to select the UV layer for the mask image.
Stencil Image
Image used as a mask. See データブロックメニュー.
Display Color

Mask color in the viewport. See カラーピッカー.

Invert Stencil (black/white icon)
Inverts the mask.