Multiplayer: Deathmatch, Last Man Standing, Control, Assault, Capture the Flag, Escape, Survival, Tag, Undefined
Social: Teams, Co-operative, Campaign
Other: Bots, Cross-platform, Cross add-on

This is surprisingly rare in commercially produced games.

For each game, at least two platform instances should be tagged with this, otherwise it's invalid (you can't have crossplatform multiplayer with only single platform).

To clarify: this means that, for example, a person on a Mac can play the game with another person on an Xbox.

This does not include the server being available on other platforms than its being played on.