Initial part to deform
The two curves are used for this curve to curve deform example
The top face is selected for deforming in all the following examples
Preview with Maintain boundary cleared and Additional faces only selected.
Under Deform Region,
the top face is selected for Additional
faces to be deformed 
.
Results - The entire model is affected. The side faces are included in the deform.
Preview with Maintain boundary selected.
The top face is selected for
Additional faces to be deformed
.
Results - Only the top face is affected.
