Kerf is the width of the blade that is doing the cutting
Lets say your width of the cut is 10mm and your blade is .1mm.
They would essentially cut out 8mm and the with of the blade would make up the extra 1mm + 1mm = the full ten
If your dxf file says you want a 10mm wide theyll deal with the kerf
There is a bottom part, this would be the case. it is basically a metal or acrylic plate that encloses and protects your wiring.
The plate that most people refer to is what holds all of the switches in its configuration, while suspending the switches inside of the case... here is an example from Xondats custom X60 keyboard.
notice how the top red part is floating to allow for space for the switches and wiring, while the bottom black and bronze section is a metal case bottom.