How rel ugc works
The rel attribute describes why one page links to another. The value ugc — short for user-generated content — says the link was placed by a visitor rather than by the publisher. It belongs on anything a member of the public can submit: comments, forum posts, reviews, question-and-answer threads, classified listings and the website field on a user profile.
Its effect is to remove the endorsement. The site is still publishing the link, and a reader can still click it, but the search engine is told the site’s editorial team never vouched for the destination. It was introduced at the same time as the sponsored value, when the older nofollow marker was split into more specific reasons for discounting a link.
Why rel ugc matters
Anywhere the public can publish a link becomes a target the moment those links count for ranking. That is the entire economy behind comment spam, forum signature spam and the fake account that fills in a profile website field. Marking those links removes the prize, which is why most established community platforms mark them by default.
It also protects the publisher. A busy discussion board will eventually carry links to sites its owner would never endorse, and reviewing every one is not realistic. The marker is how a site can host an open conversation without appearing to recommend whatever is posted in it. Being specific rather than using a blanket nofollow also tells search engines something true about the page, which is worth more than a single undifferentiated marker.
Common mistakes with rel ugc
The first is applying it to the wrong links. A page that accepts comments still has its own navigation, its own body copy and its own internal links, and those are editorial. Some plugins mark every link on a page containing comments, which is a heavy-handed setting worth checking.
The second is the reverse: leaving a community area unmarked because traffic is low and nobody has spammed it yet. Automated spam does not wait for a site to become popular.
The third is confusing it with the other two values. Use rel sponsored when money or goods created the link, ugc when a visitor posted it, and rel nofollow when neither fits and you simply do not wish to endorse. And do not think of the marker as moderation: it changes how search engines read a link, not whether the link is safe for your readers to follow.
How to act on it
Set it once at the platform level rather than link by link. Most content management systems and forum packages can apply the value to submitted content automatically, and that is the only approach that survives contact with a real community. Check what your setup actually outputs by inspecting a live comment; defaults vary between versions and plugins.
Keep moderation separate and keep doing it, because the marker does nothing about links to malware or scams sitting in front of your visitors. If your site does not accept public submissions at all, this value has no use on it — and if you are considering opening comments, weigh the moderation work honestly before you do. For the wider question of which links are worth having in the first place, see the note on dofollow links.