The id this activity has in its source system (e.g. the Strava id).
Stable activity id, ${source}:${externalId} — never a raw provider id.
Source's own sport label, e.g. "Run", "Ride". Free text by design.
OptionalstartISO 8601, local to where the activity happened (if the source gives it).
ISO 8601, UTC.
Activity metadata — the witness statement, minus its substance. The track and other streams live in ActivityStreams, stored alongside but separately (a track is large; metadata is small and listed often).