Bunkers

Bunkers (by DCoder)

Battle Bunkers allow certain infantry inside for protection and improved firepower. Tank Bunkers allow tanks to go in for the same.

Battle Bunkers (infantry garrisons)

Buildings that allow infantry to garrison inside need the following tags: CanBeOccupied=yes, MaxNumberOccupants= (max number of infantry allowed inside) and CanOccupyFire=yes.
Infantry that goes inside buildings to garrison them needs these tags: Occupier=yes, OccupyWeapon= and EliteOccupyWeapon= (all self-explanatory). However, all garrisoned units would use the [General] -> OccupyWeaponRange= instead of their OccupyWeapon's Range=, rendering any sniper garrisons pointless.

Tank Bunkers (vehicle garrisons)

Buildings that allow tanks to hide inside need these tags: Bunker=yes, NumberOfDocks=1, NumberImpassableRows=0.
The vehicles that are allowed must be:

• Vehicles with Locomotor={4A582741-9839-11d1-B709-00A024DDAFD1} (the ground vehicles one).
  
• Without Bunkerable=no.
  
• Either:
  
  • With a turret.
    
  • With a proper OmniFire=yes weapon.
    
• Drone-free.
  

Tank Bunkers improve the vehicle weapons' Damage ([CombatDamage] -> BunkerDamageMultiplier=), ROF= ([CombatDamage] -> BunkerROFMultiplier=) and add bonus to their Range= ([CombatDamage] -> BunkerWeaponRangeBonus=). The Tank Bunker usually absorbs the damage instead of the tank within, unless the fired weapon has PenetratesBunker=yes, which is usually the case on aircraft. Also, Tank Bunker protects your vehicles from Terror Drones.

Other notes

It's worth noting that standard YR AI can not effectively use infantry garrisons, due to certain programmers' laziness. That's why AI so often uses soldiers to destroy the civilian garrison building instead of garrisonning it. It might be fixable with custom AI, but I'm not sure.
Note to mappers: Look at the official Arena (2-4) map. See how close those civilian bunkers are to the player locations? Never do that! The AI will destroy them prior to building it's own base.
Garrisonning a building transfers it's ownership to you, and ungarrisonning it returns the owner to neutral. I remember making buildings stay neutral even when garrisoned (cool effect), but can't remember how I did that.

Its fixable with custom AI, there is a script tag which tells them to garrison instead of destroy and there is also a seperate tag for battle bunkers, this is YR only ;) - Allied General