G616.mp4 -

: Selected for devices or players that prefer the MP4 container over WebM/VP9 for high-definition playback.

Format 616 is a high-bitrate video stream offered to subscribers. While it provides the same 1920x1080 resolution as standard 1080p (typically format 137), format 616 is part of YouTube's "Enhanced Bitrate" tier. g616.mp4

The code in the context of an MP4 file refers to a specific YouTube video format identifier (itag) used primarily for 1080p Premium content. What is Format 616? : Selected for devices or players that prefer