It is a fallback for devices that can't handle modern codecs.
The video was uploaded during the platform's infancy. 602 mp4
Technical enthusiasts and "archival nerds" track these IDs because they reveal how platforms like YouTube manage billions of files. Format 602 is specifically a container, often delivered via the HLS (m3u8) protocol for iOS and mobile devices. It is a fallback for devices that can't handle modern codecs