http://www.pcgamer.com/play-halo-wars-2-against-xbox-one-players-thanks-to-crossplay-update/

You can now stomp on Xbox One players in RTS Halo Wars 2 thanks to crossplay support, which was added this week. But you won't be able to rank up while doing it: crossplay is only available in unranked game modes for now "for the sake of fairness and parity", developer 343 Industries said. The exception is a new game mode called Ranked 3v3 X War, which has been introduced specifically for this update.

You can also play cooperatively in Blitz Firefight (a hoard mode), Terminus Firefight (build a base and survive against waves of enemies), and the campaign. Custom matches are fair game, too.