Allows for the remote operation of the ships' doors.
Although simple in theory, doors have many uses:
Can be used to vent oxygen into space to suffocate a fire, or gradually suffocate intruders.
Can be used to quickly supply oxygen from the rest of the ship to recently repaired areas.
When combating a superior boarding party (for instance, if you had a crew complement of mostly Zoltan or Engi facing off against several Mantis), you can vent the entire ship besides the medbay or the rooms beyond it. This will quickly force the boarders to assault your medbay, where your crew will be healed while fighting. Once the boarders are in the medbay, you can safely resupply the ship with oxygen.
Upgrading impedes fire spread and intruder movement.
Can be manned with a crew member (In Advanced Edition) to give one additional upgrade on top of your current (Ex: You have level one doors, but you man it and it becomes level 2 for the duration that you man it). Also allows a 4th level only obtainable via manning.
A destroyed door system prevents remote operation of doors, but still allows crew (and boarders) to enter and exit rooms. Note that this may jam open doors you'd like closed, or jam closed doors you'd like opened.
Allows remote opening and closing of doors
Only the Rock Cruiser Layout B (the Shivan) can buy this upgrade, as it is the only ship that starts off without a door system. (It is also the only ship that lacks doors to the outside, meaning that it cannot vent oxygen without a hull breach or Lanius crew member.)
Blast Doors, slows down fires and boarders.
Improved Blast Doors, slows down fires and boarders more.
Super Blast Doors, slows down fires and boarders even more. Only available when Improved Blast Doors are manned.