The original concept was the followings:
Raiders signup to the raid until 1 day before raid start. If a raid is ”overbooked” (more raiders signed up, than needed) the leader select people, who will be there, and mark them as Confirmed. Sitting means that raider should be online, as there is a possibility, that the raid will need her/him in any part of the raid. Waitlist means that raider will not be there unless some of the confirmed staff have problems (i.e. confirmed ppl. has net problems etc.)
So basically:
Confirmed means you will be in the raid.
Sitting means, you will not be in the raid at the beginning, but be online
Waitlist means, you don’t have to be online
However use those statuses anyway you want.