https://www.gamespot.com/articles/more-nintendo-switch-online-service-details-reveal/1100-6461758/

Nintendo devoted a portion of its latest Direct presentation to sharing more information on its upcoming paid online service for Switch. The service will begin rolling out for players next week, on September 18 (September 19 in Europe and Japan), and now we know a few more details about its classic games library and other perks.

As previously revealed, while online services have been free on Switch since the console launched in March 2017, you'll need to have a Nintendo Switch Online subscription in order to play games such as Splatoon 2 and the upcoming Super Smash Bros. Ultimate against other players online going forward. Many of these games will receive additional features via the Nintendo Switch Online smartphone app. Splatoon 2 players, for instance, are able to access SplatNet 2 on the app--a service that allows them to view stats, the stage rotation schedule, and order gear to pick up in the game.