Allows for the remote operation of the ships' doors. Doors can be used to vent oxygen into space to suffocate a fire, or gradually suffocate intruders. Upgraded doors impede enemy boarders and greatly slow the spreading of fires.
When doors are opened between rooms, their oxygen levels will even out. Opening internal doors can quickly supply oxygen to areas that were recently vented.
Doors can be be manned by a crew member (in Advanced Edition), giving one additional upgrade level. This is the only way to obtain level 4 doors.
When the door system is destroyed, you cannot open or close doors. Crew can still pass freely between rooms, however.
Fires spread easily through level 1 doors, and even more easily through open doors. Fires spread extremely slowly through upgraded doors.
Enemy boarders will break down blast doors. They make about one attack per second, but the timing is slightly randomised. Doors "remember" how many hits they have taken, and never reset their "health" unless you make an FTL jump. When a door is broken, it remains stuck open for 7 seconds, after which its health is reset and you can control it again.
The only exception to this rule is when your door system is hacked. During the active hack, all your doors temporarily become enemy blast doors, and their health is reset each hack cycle (this doesn't apply to the doors on your door system itself).
The number of hits required to break down doors varies by the system level and the difficulty setting: both player and enemy doors are weaker on higher difficulties:
Hacked doors have the same strength as level 3 doors. If the door level changes while it's being attacked, it will work as you might expect: existing damage is remembered, but new damage is dealt at the resiliency of the current system level (so it's "pro-rated").