Soft Edges 
are typically used when editing photographs, paintings or pictures for PhotoStitch. The Tools give a softer (anti-aliased) edge with gradients and semi-transparencies.
Note: To see the effect of using Soft Edges 
, draw a thick line in a dark color with the Paintbrush 
, using the Brush type Very Soft Circle.
To use a Soft Edge tool
-
In the Paint Tab
of the Design Panel 
, in Mode choose Soft Edges 
. -
Choose Paint > Soft Edges.