Why scanning masks?
Film negatives are highly sensitive and therefore need proper treatment. To hold a negative in place and prepare it for scanning, a photo scanner comes with a scanning mask for your film. However, these scanning masks are usually not prepared for scanning special formats: they cover the sprocket-holes of the film, pre-defined frames and aren't comfortable to use. Their use is limited to scanning a standard landscape picture taken with a standard camera.
But what do you do with all those overlapping exposures, doubles, triples, endless panoramas and sprocket-hole images you get from experimenting with analogue photography? You need a scanning mask that isn't limited to standard frames and formats!