EyeonTips:Manual/Tool Reference/Paint
From VFXPedia
To create a mask of the area that was affected by the paint tool, instead of using a Difference Key or duplicating the whole paint node, insert a Channel Boolean before the paint tool that sets the alpha channel to black. Now all your paint strokes will be "recorded" to the alpha channel. For this trick to work with the clone brush, the clone source has to be set to a node that has a solid alpha channel, for example the node preceding the Channel Boolean if you want to clone from the current image itself.
