What is the difference in Google's treatment of 'nofollow' links within user-generated content compared to sitewide 'nofollow' links?

Google treats 'nofollow' links within user-generated content (UGC) and sitewide 'nofollow' links differently, primarily concerning how they impact a website's reputation and its ability to pass PageRank. A 'nofollow' link is an HTML attribute (rel='nofollow') that instructs search engines not to pass link equity (PageRank) to the linked page. For UGC, such as comments, forum posts, or guest blog posts, 'nofollow' links are typically used to prevent spam and discourage abuse. Website owners implement 'nofollow' on UGC links as a preventative measure, indicating that they don't necessarily endorse the linked content and aren't responsible for its quality or accuracy. Google understands that UGC is often less trustworthy and may contain spam or low-quality links. Sitewide 'nofollow' links, on the other hand, apply to all links on a webpage or an entire website. These are less common but are sometimes used for sponsored links or links to untrusted external resources. A sitewide 'nofollow' can significantly impact a website's ability to pass PageRank to other sites, potentially hindering its SEO performance. While Google doesn't explicitly penalize websites for using 'nofollow' links, excessive use of sitewide 'nofollow' can signal a lack of trust in external resources, which may indirectly affect the website's reputation. More recently, Google introduced 'rel=sponsored' and 'rel=ugc' attributes for marking sponsored and user-generated links, respectively, providing more granular control over link attribution. While nofollow prevents the explicit passing of PageRank, Google may still use nofollowed links as a signal for discovery purposes.