Canonical tags express to Google, the preferred version of a URL when duplication of the page, the page content or the URL are available to crawl. Self referencing canonical tags can be included in page code to prevent Google over-indexing multiple versions of the same page because parameters or tracking codes are passed into the URL. Cross-domain canonical tags can reference a page on an entirely different domain that hosts the same content as another page on a different domain. These are often used when a news network publishes a story to multiple websites for local news outlets that are part of a larger conglomerate. In general, it is considered a Best Practice in SEO for all pages to at least have a self-referencing canonical tag, unless the page is canonical to an entirely different page. Canonical tags should be consistent, and should not conflict with other signals, such as server enforced URL rules, sitemaps and internal links. They should also not chained or inconsistent with other canonical tags on the site.