1. This drop down allow you to chose from 3 ways to combine new mask areas with the mask that is already present. These are Replace, Add and Subtract.
2. The Invert option, when enabled, will invert the drawn mask before it is merged with the current mask.
3. The Level slider controls the strength of the mask from 0 (no mask) to 100 (full mask).

1. Replace mode selected.
2. A rectangle mask at 100% strength.
3. A rectangle mask at 50% strength.
4. Notice that the mask at 50% strength replaced the mask at 100% strength where it overlapped it.

1. The Add mode is selected.
2. A rectangle mask at 100% strength
3. 2 rectangle masks at 50% strength
4. Notice that in Add mode the 50% strength mask added to the 100% strength mask remains at 100%
5. Notice that in Add mode the 2 50% strength masks rectangles where they overlap combine to make a 100% strength mask area.

In this example we have started with a full mask covering the entire image.

1. Subtract mode selected.
2. A rectangle mask at 100% strength completely removes the mask.
3. 2 rectangle masks at 50% strength subtract from the existing mask.
4. Subtracting from a full non-masked areas resulted in no change.
5. The area where the 2 50% strength masks overlapped results in the mask being completely removed.

The Invert option can require some abstract thinking to use. It is never required to use it but the feature exists for those building complex masks.

1. The Invert option is selected.
2. The Add mode is selected. Invert can be used with any mode, Replace, Add or Subtract.
3. 2 rectangle masks at 50% strength are drawn. Since the Invert option is active the rectangle area drawn is the area that is not affected but everywhere else is. Where the mask strength is 50% is the area where only one of the masks inverted rectangles were drawn.
4. This area while is seems like an intersecting area is actually the area that had not mask applied from either inverted rectangle.
5. The large area at 100% strength is were both rectangle masks occurred.