[modification]:

The [modification] toplevel tag

This tag describes a multiplayer modification. A modification is practically a bunch of events which get included into the scenario if the modification is enabled.
The following keys/tags are recognized for [modification]:
* id identifier for the modification. Must be unique.
* name the name of the modification, as dispalyed to the user
* description a brief description for the modification
* allow_scenario a list of scenario ids. Only the scenarios with matching ids will be allowed to be played with this modification.
* disallow_scenario a list of scenario ids. Only the scenarios with matching ids will not be allowed to be played with this modification. Cannot be used in parallel with allow_scenario.
* allow_era same as allow_scenario, but for eras.
* disallow_era same as disallow_scenario, but for eras. Can't be used with allow_era.
* allow_modification same as allow_scenario, but for modifications.
* disallow_modification same as disallow_scenario, but for modifications. Can't be used with allow_modification.
* ignore_incompatible_scenario a list of scenario ids. The scenarios with matching ids will be considered compatible with this modification regardless their dependencies.
* ignore_incompatible_era same as ignore_incompatible_scenario, but for eras.
* ignore_incompatible_modification same as ignore_incompatible_scenario, but for modifications.
* require_modification a boolean value; if set to yes, all players have to have this modification installed to join the game. Default no.
* [event] any [event] children written inside the [modification] tag will get included into scenarios that are played with this modification enabled. See EventWML.

[era]:
the appropriate allow/disallow/ignore keys should be included here too

[scenario]:
same