While I was looking for a cute image of North to post in the chat room, I came across this image
I remember in Season 2 Episode 8 - Chinatsu Unrivalled (I think), Toshinō Kyōko gets everyone to help her with her doujin work by doing some shading. From what I remember, they place the darker sheet overon top of the outlined doujin pages.
So what exactly is this sheet? whatWhat is it used for? isIs it something doujin artists use to supplement what a professional would use, or is it used by professional mangakas as well?