Overlays are datasets with transparent backgrounds that contain foreground data used to augment other SOS datasets.This particular dataset overlays a black mask over all the land on any dataset that you are viewing.
Overlays can help provide context for your audience and make the datasets easier to understand, as well as provide information that may be useful to illustrate related global ideas.
Overlays are accessible on the SOS Remote app for iPad/iPhone/iPod Touch on version 4.0.2 and above.
Simply tap the Overlays button in the upper right hand corner of the Presentation tab to open the list of available overlays. Note that Overlays may also be pre-programmed into a playlist as a layer.