Cross-domain canonical links are often used when the webmaster mixes content from his website with an external website.
These URLs can set an interdomain canonical value and show that the original URL is the canonical version.
What Does “Canonical For External URL Issue” Mean?
A Canonical For External URL Issue means a particular URL has a canonical tag pointing to an external URL. External means that the URL is in a different domain or subdomain, i.e., a different resource.
It is called interdomain canonical.
Using this link to check for more information about consolidating duplicate URLs:
What triggers this issue?
The issue emerges due to a canonical tag pointing to the URL of a page hosted on a different domain. The tag can be specified either in the HTML or in the HTTP header. The external URL is defined as being in a different subdomain and not belonging to this site.
An issue may occur if the webmaster has mixed the content of two sites on one page. It can also be an accidental error if the webmaster has several sites under development at once. Human error or an intentional act can be the cause of the Canonical For External URL Issue.
How to check the issue?
Use the Google Search Console webmaster tool to find this error on your site. A crawler can also help you by verifying if the site has a Canonical For External URL Issue.
Detect Canonical pointing to external URL on your website and go ahead to analyse the other issues on it!
Check not only the issue but make a full audit to find out and fix your technical SEO.
Here is the Ask Google Webmasters episode, where you can get more data about checking the issue with Google services.
Why is this important?
The services recognize this error as non-critical and requiring review. It means that there is no need to take any action to correct the error. Just be aware that this phenomenon has occurred on the site and may affect its performance in the future in conjunction with other problems.
If the URL of your website contains an interdomain canonical tag, it indicates to the search robots the need to combine link signals with the canonical URL. In an example, it would look like this: if page X has external links pointing to it, page Y will get the value for those links.
How to fix the issue?
Canonical points to external URL occurs accidentally due to the webmaster’s actions or by the webmaster’s intention. That is why this error will be displayed as low criticality in crawlers.
The webmaster must double-check to which URL the canonical tag of a particular page points to fix the issue.