The UGC tag, rel="ugc", is one of the three link attributes Google introduced in 2019 specifically for links that a site's own team didn't write or place: comments, forum posts, guestbook entries, reviews, and any other content submitted by visitors rather than the site's editorial team.
It exists because user-generated content is exactly the area a site has the least control over, and historically the area black hat link builders exploited most, planting links in blog comments and forum signatures across thousands of sites at scale. Tagging links in these areas as UGC tells search engines the site isn't personally vouching for them the way it would an editorial link, which protects the site from being held responsible for spam a visitor added rather than content the site itself published. Most modern comment and forum systems apply the tag automatically to user-submitted links by default, but it's worth confirming on any custom-built comment or community feature rather than assuming it's handled.
