Super Spy Deluxe
In order to change a planet's friendly code, you must have at least one of your ships in orbit performing the Super Spy mission, with fuel aboard. That ship will have a 20% chance of changing the planet's friendly code. For each additional ship you have in orbit (with fuel aboard), your chance of changing the planet's friendly code increases +20%. So if you have 2 ships, the chance of changing the FC will be 40%. If you have 5 ships in orbit, the chance will be 100%, which means that you are guaranteed to change the FC.
Note: only 1 ship is required to be performing the Super Spy mission in order to change the FC, and gain the increased +20% chances. All that is required from the other ships in orbit is that they have fuel aboard.
Ion Pulse: Whenever you change a planet's FC, there is a 20% chance that it will emit an ion pulse that will de-cloak any ship in orbit. In order to do this, the planet must have at least 30 defense posts, and 10 of them are sacrificed to emit the ion pulse. If you change the planet's FC to 'MFx' (where x is any character), it will always trigger an ion pulse, if possible.
Changing a Planet’s Friendly Code
Changing a planet’s friendly code will prevent the planet from attacking any ship with the same friendly code. It will also allow ships with a matching FC to beam up fuel, minerals or supplies from the planet by using one of the gather missions. (This is not recommended at a planet with a starbase however, since if the starbase has the Force Surrender mission active, any ships with matching FCs will surrender to it—unless they have the Super Spy mission active).
Minefields will have the same friendly code as the planet closest to their center. Any ship with the same FC as the minefield can travel through the minefield safely.
Special Friendly Codes
There are interesting things you can do by changing the planet to the following friendly codes:
"Beam up money". Causes the planet to send its MC to foreign ships in orbit (including cloaked ones). You can steal much money from an opponent this way.
Changing the friendly code to NUK will cause it to attack fuel-less ships in orbit (but not Birdman ships). So, if you have a non-bird ally, they can bring a fuel-less ship into orbit, and have it be attacked by the planet, while not having to fight any defending ships there.
If a starbase is present and has a ship queued to be built, changing the starbases's friendly code to PB1 (or PB2, PB3, etc.) will cause the enemy to build a ship there, spending priority points to do so. This gives you the possibility of capturing that ship after it is built, since ships are built without fuel to start. To prevent the enemy from putting fuel on the newly-built ship, you must first destroy the starbase (on the same turn that the ship is built) and conquer the planet. Afterwards you can tow the ship to one of your starbases where you can force a surrender and capture it.
Note that capturing the target ship will not work if the starbase mission is set to "refuel" and the ship receives fuel before combat begins.
However, changing a planet's FC to PBx is still useful because you can force an opponent to waste priority points on a ship they didn't need. You can also destroy the ship that is built easily because it won't have any torpedoes or fighters on board, which will win you priority points for destroying it.
Changing a planet's friendly code to this will trigger an ion pulse every time, as long as the planet has at least 30 defense posts. This can be useful if you want to deliberately trigger an ion pulse. An ion pulse will de-cloak all cloaked ships in orbit (which is something you do not want, normally). However, if you suspect an enemy has its own cloaked ships in orbit, it can be useful to deliberately trigger an ion pulse to de-cloak them (in addition to your ships). The de-cloaking ion pulse occurs before ship movement.
MFx friendly codes will change the minefield FC for all of an opponent's minefields (see below).
"Dump parts from starbase". Not very useful in most cases, since people don't usually store parts (engines, beams, torp tubes) at a starbase. But if they do, this will recycle them and place the recycled minerals on the planet's surface.
Changing a planet's friendly code allows you to control enemy minefields, which take on the FC of the nearest planet belonging to that player. Any ships with the same friendly code as a minefield will not hit any mines.
If you set an MFx friendly code on a planet (where x is any character), it acts as friendly code for all minefields belonging to the player who owns the planet. The global MFx friendly code is set by the highest ID# planet with an MFx friendly code. So, if the player has a different MFx code set at a planet with a higher ID#, that will be the global friendly code instead.
If you can control the planet with a highest ID# belonging to an enemy, you can control the global minefield friendly code for all of his minefields.
Super Spy Command
With Campaign mode enabled, Birds have access to Super Spy Command, which prevents enemy starbases from launching fighters. This is accomplished by setting the planet's friendly code to NTP.
Note that one of your ships needs to have its FC also set to NTP, which means it will not attack, even if it is uncloaked.