About combining masks with tracking behaviors
When you add a mask to an object that has applied tracking data, the data is applied to the mask. In the following image, the foreground object is tracked to four points on the background clip, but still must be masked to fit into the picture frame.
By masking the image being tracked, you can include only the portion of the image you want to fit in the frame.
Although the mask is attached to the foreground image, you might need to animate changes in the mask’s position and scale to accommodate changes in the background clip over time.
Note: As an alternative to using masks, you can also crop an object with applied tracking data using Crop controls in the Properties Inspector.
You can also track the control points of a mask to a clip or apply existing tracking data to the control points of a mask. For more information, see Track shapes, masks, and paint strokes.