A Supplemental Listing is a almost like having a secondary listing of a webpage.
Here is an example of how a Supplemental Listing appears on Google:-

Generally Supplemental Listing will display related individual web pages relating to the Keyword, Keyword Phrases searched with Organic Search Engine Optimization listings.
Advice: A Supplemental Listing is good because it shows that this particular webpage is also listed and users may also click on this link
Other search engines, (as well as Google), may also use Supplemental Listings.
View the sections Google Information and Organic Search Engine Optimization.
Google Knowledge. Google Information
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