Canonical points to external URL
This means that the URL in question has a canonical element pointing at an external URL (a URL on a different domain or subdomain). This is known as a 'cross-domain canonical'.
Why is this important?
Cross-domain canonicals are often used when a publisher syndicates content from another source - they can set a cross-domain canonical to indicate that the original source URL is the canonical version.
If Page A is canonicalized to Page B, it is explicitly marking Page B as it's canonical. This acts as a strong signal to search engines that these two pages should be considered duplicate, and that Page B should be prioritised.
If a URL contains a cross-domain canonical, this indicates to search engines that they should consolidate link signals to the canonical URL (i.e. if Page A has some external links pointing at it, Page B will get the value for these links).
What does the Hint check?
This Hint will trigger for any internal URL which contains a canonical link element (either in the HTML or in the HTTP header) which specifies an external URL as the canonical URL. An external URL is defined as being on a different subdomain.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it had any of the following;
Canonical link in the <head> to a URL on a different root domain:
Canonical link in the <head> to a URL on a different subdomain.
HTTP Header canonical link to a URL on a different root domain.
HTTP Header canonical link to a URL on a different subdomain.
Why is this Hint marked 'Insight'?
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing.
Canonicals implemented in this manner are usually deliberate, so they are flagged in Sitebulb in order to draw your attention to the URLs that have cross-domain canonicls, so you can double check that they have been implemented correctly.
While Sitebulb can check if a canonical tag is valid, it has no way of knowing for sure that Page B should be the canonical URL for Page A.