For example, if you're playing on Xbox One and a friend has bought ARK through the Windows 10 Store, you can play together.
Congrats! If you are playing on the Xbox One and have a friend who plays on a version from the Windows 10 Store, then you’re in luck.
So technically PC and Xbox One are cross-platform, although you can only play together with a Windows 10 version of the game.
Don't go buying it on Steam expecting the same result