Every attendee who has taken a spot on an upcoming, ongoing or past shift carries a status on their reservation. There are nine, and this article explains what each one means and how a reservation moves between them.
1. Reserved. The attendee holds a spot on an upcoming shift. Admins are notified when somebody reserves.

2. Attendance confirmed. The attendee was asked to confirm within 48 hours of the shift start time, and confirmed that they are coming.
3. Checked-in. The attendee reserved a shift and was checked in.

A reservation showing Checked-in can belong to a shift that is still running or one that has already finished.
4. Not logged. The attendee reserved a shift, the end time has passed, and they were never checked in. This status also stays put indefinitely until the attendee claims hours or an admin logs them.
5. Hours logged. Five routes lead here:
Attendees are notified once hours are logged.
6. Absent. Five routes lead here, and all of them end with an admin marking the person absent:
Only admins can mark somebody absent, and the attendee is notified when it happens.
7. Hour claim pending. Somebody submitted an hour claim, either for a past shift they had reserved or for one generated through the general Claim Hours feature. Admins are notified of a new claim and can approve or decline it. Approving moves the status to Hours logged.
These last two appear only on opportunities with Instant Book disabled, because that is the only setting where a reservation becomes a request first.
8. Shift request pending. Somebody asked to reserve a shift and the request is waiting for an admin to approve or decline it. Admins are notified of a new request. A pending request does not consume a spot.
9. Shift request declined. Somebody asked to reserve a shift and an admin declined the request.