A National MLS is a nation-wide pool of property listings under a common set of rules and technology standards and available from a single source. In many ways DDF operates as a national MLS In Canada. No equivalent service exists in the U.S., however.
Although there is no National MLS, there are alternatives that still allow you to integrate national listings, such as Organic Listhub Integration(https://realtyna.com/listhub-integration/). Please see our blog for more details: https://realtyna.com/blog/how-can-get-national-mls-feed/