Apple Color 1.0 - Manual (Page 304)

304 Chapter 15 Geometry
To feather the edge of a shape:
1 Increase its Softness value.
The Softness parameter applies a uniform feathering around the entire shape. This also
reveals a pair of inside and outside shapes that represent the inner and outer
boundaries of the feathering effect that’s applied to the shape.
2 If necessary, adjust the shape’s inner and outer shape to create the most appropriate
feathering outline around the perimeter of the shape.
This lets you create irregularly feathered outlines when you’re isolating a feature where
one edge should be hard, and another feathered.