Each minefield has an own, "local" friendly code (as above). In addition, you can define a "universal" minefield friendly code. For this, set a planet to mfX, where X can be any character. Ships can then use this friendly code to safely pass all your minefields.
This is useful when you want to grant a (not-yet-)ally free flight through your minefields without granting him access to your planets.
mfX is not considered a special friendly code.
