Sponsored and nofollow

Rel attributes are written in the link's HTML and are invisible to the reader. nofollow arrived in 2005 as a defence against comment spam, and in September 2019 Google added 2 more precise variants: sponsored for paid and commercial links, and ugc for user-generated content such as comments and forum posts. Since March 2020 Google has treated all 3 as hints – signals that feed into its assessment, not orders that are followed blindly. Other search engines read the same attributes, but Google's guidelines set the standard in practice.

For affiliate marketing the message is simple: Google recommends rel="sponsored" on affiliate links and accepts nofollow as an alternative. The marking tells the search engine that there is money behind the link, so it does not count as an editorial recommendation in the link graph. Unmarked paid links on a large scale are treated as ranking manipulation and can trigger a manual action against the site.

The attribute changes nothing for the user or for tracking. The link works, the click is recorded and the commission is paid exactly as without the attribute – rel is only read by crawlers. So there is no financial reason to leave it out, and good reason to use it: it protects the site's visibility, which all affiliate income ultimately depends on.