Its the waterproof material same/similar to the Apidura waterproof packs. And need to glue/join it to the steel (italian Columbus I'll have you know... built by a very posh man that hardly smiles, I'll have you know) top tube.
Been using velcro previously but that doesn't last too long and have decided to go all out. Just don't wanna end ip with a mess of bag and frame...

Mind the background wallpaper... Not sure who on earth would put that there...