These module foams can be used between the PCB and Plate. It does have a sticky side, most people will stick it on the plate, instead of the PCB. It's really up to preference in the end. Each foam is perfectly shaped for one switch slot, so this foam is perfectly compatible with MOST PCBs layout. 

And since, this foam is made out of PORON, it greatly improves to mute and decrease the hollowness.

"Personally, I would only apply these on the outer keys like F-row and the bottom keys (ALT, CTRL, not the spacebar) to improve on hollow cases."