These foams are specifically for the Navigation keys (home, delete, pgup, etc) and Arrow Keys (up, down, left, right). Placing these between the PCB and Plate.
Since, it's made out of PORON, it greatly improves to mute, and help with hollow keyboard cases.
These are cut out to fit very certain layouts, mostly full size keyboards. Cannot fit 1800 layouts, unless you cut it on your own.