Channel ID

Find it in Studio under advanced channel settings, or read it out of the address bar on any youtube.com/channel/ link. It is what the Data API, third-party analytics tools and a channel's public RSS feed all expect.

Handles look like the obvious identifier and are the wrong thing to store, because a handle can be changed and later picked up by somebody else, at which point your saved reference points at a stranger's channel. The ID is issued once and never reused.