FAQ - Aperture
What is a Mask?
The Mask determines the shape of the object. The mask should be solid in parts of the image where the photographed object is, and transparent in parts of the image where the studio walls and floor are.
A Machine Learning algorithm takes a first guess at the mask, but it must be approved by a human user before it may be used. This is called the Machine Learning Mask.
Once a mask has been approved, it becomes the default for new images made either by the editor or the batch processor for that capture.
Results from Machine Learning get better over time.