The session this registration is for, or null for an activity-level registration that covers the whole activity rather than one session — e.g. a team's season "league registration" (see kind: leagueSeason). Per-session registrations (class drop-ins, the comp spots fanned out to a team's members for each match) always have a session.

Timestamp when this registration's status was set to confirmed — either at register time for a direct registration, or at promote time when a waitlist row flipped from waitlisted / offered to confirmed. Null on waitlisted / offered rows that haven't been promoted yet, on cancelled rows that were never confirmed, and on historical rows pre-dating this column (consumers fall back to createdAt in that case).