Boolean operations combine any set of shape layers through one of four formulas: Union, Subtract, Intersect, and Exclude. Boolean groups are treated as a single shape layer and share fill and stroke properties and can be combined with other boolean groups through subsequent boolean operations.
Union combines the selected shapes into a boolean group. If the objects overlap, the new shape’s outer path consists of the composite of its sublayers’ paths minus any segments that overlap. The stroke would then be applied to that outer path ignoring any path segments which overlap each other.
Subtract is the opposite of Union. Subtract removes the area of a shape or set of shapes from a base shape. Only the bottom shape layer is solid, the rest are subtracted from it.
Intersect creates a boolean group whose shape consists only of the overlapping parts of its sublayers.
Exclude is the opposite of Intersect. Exclude shows only the areas of its sublayers that do not overlap.