Friendly Codes

BDF: Beam down fuel (All but 60kt)
BDM: Beam down metals and money
BDA: Unload all guests and cargo except Ordnance
ORD or BUO orders the ship to beam up ord units from any base under the ship that you own or has a matching friendly code.
BDO Beam Down Ord (all but 500)
NCH: Any ship with this code will not enter a warp chunnel or Jump via a jump point
BDP moves troops, colonists, highguard and money to the planet's surface
BUS Beam up supplies
BUR Beam Up Repair units
BDP Beam down people and money (useful for Cyborgs when using their Assimilation Beams)
RFD RFT RFM RFS can be used by Fuel Refinery ships to control what goods will be used to make fuel
JOE can be used by a ship with a Jump Point Generator to ensure that only escorting ships and wings will jump with the ship

Base Command Codes:

TNT - you can destroy a base by setting the command code of the base to TNT.
This requires that a ship be in orbit and no colonists on the base.
GBA - Give Base Away. Your base will be given to any other race with a base on the planet. It will go to any friends you have first, if you have no friends it will be given to your enemy. All your assault units, fighters, special race buildings and high guard will vanish.