Filters are vaguely similar to things like mountpoints overlaying portions of the filesystem. E.g. in Linux you might have files in /d1/d2/{f1,f2,f3} in the ro

Filters are vaguely similar to things like mountpoints overlaying portions of the filesystem. E.g. in Linux you might have files in /d1/d2/{f1,f2,f3} in the root filesystem but you also have a mountpoint of a 2nd filesystem on /d1/d2 that completely changes the visibility / contents of d2. Filter drivers can do similar things (although they are not actually independent mountpoints).