The plugin advertises h264 and av1 and encodes every video rendition the node hands it, each as its own fMP4 track (av01 + av1C for AV1, built from the encoder's sequence header with a port of obs-outputs' AV1 config parser, which libobs doesn't export). AV1 packets are already temporal units; only the OBUs an MP4 sample must not carry are dropped. The node now hands the plugin every video rendition record in rkey order (the first is the source), keeping the ones in codecs the plugin lists in client.supported_codecs; a plugin from before AV1 lists none and gets H.264 only, or an "update the plugin" problem when there's no H.264.