System event group

Hi.
I'm reading Gecko event dispatch code.
While tracing gSystemEventGroup, I found that "keydown", "keyup", and
"keypress" event listeners
are registered in the system event group. I'm at a loss as to how I
understand this group and havig
some questions.
What's the difference of these listeners from those of default event
group?
What if these listeners are registered in the default group?
Bugzilla reveals that XBL uses system event group. Is this related
with above "keyxxx" event?

Thanks

Cho.