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 'Advisory'?
In Sitebulb, this Hint is Advisory, as it does not represent a clear issue. 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.