Multiplayer: Cross add-on
Allows playing multiplayer games across expansions, so players with expansions can play with people who don't have them without any additional work (such as no need to start the game with expansions disabled) nor notable detriments to expansion owners.
Alternate name: Cross expansion
The first video game about Multiplayer: Cross add-on was released on September 20, 2004.
Other: Bots, Teams, Cross-platform, Cross add-on
For games with no standalone expansions, this effectively means that you MUST be able to play as or with whatever the expansion adds but those without the expansion can't. As in, if the expansion adds a faction, units, or whatever else, you should be able to play with them even though your opponent can't. If the expansion adds only maps or such, then this tag does not particularly say anything useful.
Regardless, it is quite common to require all players to play with the same expansions installed. If one has more than the other, they commonly have way to limit themselves only to the content of the previous expansions, which effectively removes the use of the later expansions and invalidates the use of this tag.
If the expansion owner is required to use only things that are available to those without the expansion, then this tag is not applicable. This refers to the "no detriments bit", for example, a player with an expansion that adds a new character type can play with that character even in games with people who don't have the expansion that adds it.
A more recent variant of this is that the expansion owners can invite people to play on expansion-only maps or similar, essentially giving a demo of the expansion content.