This feature can enable modders to create diverse lore and background for different types of base defense missions (that are technically already possible with new features like
https://openxcom.org/forum/index.php/topic,11429.0.html and
https://openxcom.org/forum/index.php/topic,10808.0.html)
Currently, the game uses can spawn alien mission for base defense encounter, that specify the UFO to "land" near the base. Battlescape encounter uses base to generate the map and `alienDeployment.data` section of the landed UFO to generate enemies (race taken from alien mission). But the briefing data is hardcoded and can't be modded.
Possible design - make a new property in `alienDeployment` rule to override vanilla logic and use briefing data from UFO's `alineDeployment.briefing` (currently, it is ignored), like it is done for terror missions.
