There are three different types of event bus lines:
The term "relative" simply means that the device provides position data that's relative to the last location it reported. This is typically the method that mice use.
An "absolute" bus line is used with devices (e.g. touchscreens) that provide position data at absolute coordinates.
Finally, a "keyboard" type of bus line is one in which some sort of keypad device provides codes for every key press and release.