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(See Blending modes)
This was one of the most exciting features to appear on Photoshop 7.0 and is located on the toolbar where the airbrush tool used to be. The healing brush is a glorified Clone tool and does a fantastic job on some things and makes a hash of others.
I would suggest that until you are familiar with this tool you can always try it first and if it doesn’t work, undo the action ...