A Clerk session token carries sid (session ID), azp (authorized party — your app's origin), and sub as the user ID.
If you're following an older tutorial, watch for this: Clerk's Session Token JWT v2 moved organization-related claims (org_id, org_role, org_slug) out of top-level fields and nested them under a single o object instead (o.id, o.rol, o.slg) — code written against the v1 shape will silently get undefined for the old top-level fields. The o claim is only present at all if the user has an active organization.