Metrics is the unit's latest utilization: its own live snapshot when it keeps one (a run-target's heartbeat wins), else the newest sample from the series for the SAME source. Absent means nothing is known about this unit's load — which is deliberately not the same as a reading of zero.
Queued is how many renders are waiting on THIS GPU's own lane in the org's gpu-jobs queue. BYO units only — an agent run-target dispatches, it does not queue — and omitted when nothing is waiting.
Sessions is how many agent sessions are open on this unit. Always present, and 0 for a source that cannot host agent sessions at all — a fact about that plane, not a gap in the reading.
Source is the plane this row came from: "agent" (a linked run-target), "byo" (a worker or cluster the org dialed in) or "visor" (a machine Hanzo provisioned). It is half the row's identity, and it says which face owns the unit — /v1/agents/targets, /v1/visor/fleet/workers, /v1/visor/machines.
Status is liveness in the SOURCE's own vocabulary, because each plane decides it differently: a run-target's is derived from its heartbeat, a BYO worker's is online/offline on the 90s window, a BYO cluster's is "attached", and a Visor machine's is the provider's word for its lifecycle state.
Unit is the SOURCE's own id for this unit — a run-target id, a BYO worker id, a Visor machine name — so a row links straight back to the face that owns it. It is unique within a source, not across them: two planes may mint the same id, which is why (source, unit) together is the identity.