7. The Menus / 7.7 The Filter Menu
This filter is used to remove dust, scratches and fluff from a scanned image. In order to do this all small objects and fine structures are filtered.
Aside from the usual filter settings there is the value Threshold. This value controls, how strong a disturbance has to be in order to be removed. A value of 0 removes all disturbances and a value of 100 removes nothing.
You will get better results, if you are selecting the disturbances by a (soft) lasso before.