You're probably correct on mode. Note there is an option for Dust and another for Spot. Somehow the program knows sensor dust from a spot in the frame, for example a fleck of mascara under a model's eye as opposed to a chunk of crud on your sensor, and won't remove the "spot" of mascara if you have the tool mode set to dust. However, since a spot is more generic, it will remove almost anything you place it over much like the healing brush in CS. Now for dusts, here's the neat trick: since dust usually sticks in one place form frame to frame, once you have all your dusts encircled -- the tool will say something like 8 out of 8 -- then you can copy and apply to all other images, removing all those same dusts from all your captures at once. In use, it's very cool. Note that the tool does a very good job so is likely doing some heavier math processing, so once a spot or dust is encircled it may take a few seconds for the software to render the removal.