When offering vastly similar contents in web pages, Google will consider them as duplicate content and index only the web page that best suits the search query. Due to this procedure, the webpage which has to be indexed is missed by Google.

For instance, http://www.example.com/products?trackingid=car
and http://www.example.com/products?categoryid=blackcar are duplicates of http://www.example.com/products. When the query is for black color, the http://www.example.com/products?categoryid=blackcar is indexed instead of http://www.example.com/products.

To overcome this issue, Google has introduced a new HTML called “link”. By adding a
element to the site code, webmasters can suggest identical pages to Google.

Copy this link into the section of all non-canonical versions of the page, such as http://www.example.com/products?trackingid=car and http://www.example.com/products?categoryid=blackcar.

The result of this approach is that the crawler does not need to crawl the duplicate pages to identify the canonical relationships.

Remember, you can suggest canonicalization only across sub-domains and within a domain, and not across domains.