Yes it it's an abrasive net, Abranet from Mirka. It is an open weave fabric that works with a dust extraction system.

It comes in all sorts of formats to match a multitude of hand sanders, orbital sanders.

It started life in Mirkas automotive product ranges, but what do you know, painters of all strokes can use it. From 40 super coarse through standard residential painter grades 80, 120, 240, 320 on to the 000's for polishing with abrasive pastes.

Now you know :)