One credential the node signed for this DID out of what its parent holds.
credential_jwt is a compact JWS, ready to attach to an outbound message as
application/vc+jwt — the same shape GET /api/v1/credentials returns, so
the wallet parses it with no special case.
It exists nowhere but the join reply. The node stores nothing about it: a credential belonging to a connection has the lifetime of that connection. There is no endpoint that will hand it back, and losing the join reply means rejoining to be issued a new one.