In the current climate of multiseat, videocards are the basic building block of each seat - that is, if you desire simplicity in multiseat. Two video cards means two seats. Three video cards, three seats. With that being said, the easy way to understand drm, or the Direct Rendering Manager, is as a kernel module which manages these video devices.
This is why in EasySeats when you assign your second or third or n video card to another seat the video card address represents as drm/card#.(insert number)
Freedesktop.org has a great write up about drm in much more detail: dri.freedesktop.org/wiki/DRM/