What is CPN in youtube video

Question

The context menu which appears after RMB click has one option named Stats for nerds after which another menu appears on the left top corner.

One of the parameter associated is CPN. What does that mean?

Answer 1

As answered officially on reddit thread:

CPN is just a string we use to identify the playback; if you have problems with the playback, developers can use it to look up log/stats information on the playback so we can figure out what went wrong.

and in the same comment tree:

Yes, CPN is just a unique playback ID, computed on the client; stands for "Client Playback Nonce."