The source's key, as its frames carried it in BridgeMediaFrame.SourceID.
OptionalSourceThe source's human-readable name ("Ada's camera") as the driver names it when the source ends, like BridgeMediaFrame.SourceLabel. It can differ from the name the source's first frame carried (the person was renamed, or had no name yet), so the engine names an ended source as the model was told it, not by this.
The plane the source was on: a camera (video-in) or a shared screen (screen-in).
An inbound video source that stopped: the person stopped letting agents see them, left, stopped sharing or turned their camera off, or the driver moved the model's view to another source. A driver that sends camera or screen frames reports it so the host can tell the model, which would otherwise go on describing the last frame it saw.