Alternate URL has no incoming internal links
This means that the URL in question has no incoming anchor links, and it was only found via the alternate link element.
Why is this important?
Any URL can declate 'alternate' URLs for itself, which should be considered as equivalent content in some other representation. For instance, the mobile version of a page may be listed as an alternate, and this can be set in the <head> or in the HTTP header.
This Hint points out that a URL is listed as an alternate, yet has not been found in the main website architecture (i.e. it has no internal links). In a lot of cases, this is perfectly normal - as such, there is potentially nothing wrong with these URLs, Sitebulb is simply notifying you that they are there.
What does the Hint check?
This Hint will trigger for any internal URL that was found by the crawler via the alternate link element, and has no other internal anchor links.
Examples that trigger this Hint
The Hint would trigger for any internal URL that was found by the Sitebulb crawler during the crawl (i.e. not found in XML Sitemap, Google Analytics etc...), which has 0 incoming links from internal pages.
Why is this Hint marked 'Advisory'?
In Sitebulb, this Hint is Advisory, as it does not represent a clear issue. There are plenty of common situations where it is deliberate for alternate URLs to not be a part of the 'user-facing' website structure.
However it is worth double checking that there are no such alternate URLs that actually should be linked through the main website. For example, you may have a small subset of translated pages which are linked to on their parent page (and also listed as alternates), but one of the translations was missed so there is no anchor link that a user can see rendered on the page.