The standard for most compressed video distribution, including MP4. Here, 16 is defined as "Reference Black" and 235 as "Reference White". Why Does MP4 Use 16-235?
Most video (the codec usually inside an MP4 file) defaults to limited range for archival and YouTube distribution. This standard originated from analog television, where "footroom" (0-15) and "headroom" (236-255) were reserved to prevent signal interference or "clipping" at extreme ends of the spectrum. 235 mp4
In the digital world, an 8-bit signal provides 256 possible values (0 to 255). including MP4. Here
If you need to deliver a file directly from a camera without editing, 16-235 is often the safer "necessary" choice to ensure it looks correct on all displays immediately. 235 mp4