Sabitoru Rust Eraser is the 'Eraser' or a Gummy Stone. The name comes from what this little flexible piece does. So you can use it to remove the patina and rust from knives and make scratch marks disappear.
The Medium grit in the shop is the most used in this series.
For me, however, the essential thing this product does really well and literally in seconds is cleaning the surface of the whetstone. You just put a little water on the stone and go through the surface, and it's ready to use again. Please Note: This does not flatten the stone; it removes metal and other dirt stuck to the surface.