rel="ugc" was introduced by Google in 2019 alongside rel="sponsored". It tells search engines the link was added by a user, not the site owner.
Like nofollow, it's treated as a hint. Sites can combine values, e.g. rel="ugc nofollow".
Why it matters when you launch
Links in comments, community threads, and self-submitted profiles are usually marked ugc. They're useful for discovery, but aren't a substitute for editorial links.