In the context of the Dynamic Adaptive Streaming over HTTP (DASH) protocol, YouTube assigns unique numeric codes to every available quality and codec combination for a video. "133" is the standard identifier for: Container: MP4 (.mp4).
For a deeper dive into how the MP4 container functions as the foundation for these streaming formats, watch this overview: What is MP4 File? | MP4 Explained | MPEG-4 Part 14 LearnEveryone YouTube• Apr 15, 2024 133mp4
While "133mp4" is widely known in video engineering, the sequence "133" also appears in specialized academic literature related to video data: In the context of the Dynamic Adaptive Streaming
Typically encoded with H.264/AVC (Advanced Video Coding). 133mp4