The visualization mode for the masks. Each bone, curve, bone group or curve group has a specific mask area on the mesh. This mask defines areas where generated morph targets can be active. They can be used to filter out deformations in undesired areas. For example if you rotate the left arm, you don't want the right arm to deform. The mask for the left arm can be setup in a way that it only includes vertices around the area of the left arm to enforce this.