The document discusses rel="canonical" tags, stating that they are a hint rather than a directive that search engines strongly honor. It notes search engines will take the canonical tag into account alongside other signals to determine the most relevant page for a topic.
50. Is rel="canonical" a hint or a directive?
It's a hint that we honor strongly.
We'll take your preference into account, in
conjunction with other signals, when
calculating the most relevant page […]
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