Final answer:
Feathering adds a soft edge to a selection in imaging software by creating a transition zone of pixels that blend with the background, with the pixel amount determining the effect's subtlety.
Step-by-step explanation:
When modifying a selection in image editing software, adding a soft edge to a selection based on a pixel amount is achieved through a process called feathering. Feathering creates a transition zone around the border of the selection where the pixels gradually blend with the surrounding pixels, resulting in a softer, more natural-looking edge. The specific pixel amount set for the feather radius determines the size of this transition zone, impacting how subtle or pronounced the soft edge effect is.