Right. And some expansion bays have special connectors that attach to the
add-on device, and slide in and lock in the open bay. Others, you have to
open the case and screw the add-on device to the chassis.
When you add a device you may have to provide: a pigtail for additional
power connector, an interface cable, or even an additional interface card.
Just depends what you have available on your PC and what you're adding to
it.
Dave