A global group is a bunch of flight groups together. It is used for combining more than one flight group into one. For example if you want all the Imperial fighters to be destroyed, but don't want the transports destroyed, make all the fighters in one global group. Then tell the guys to attack the global group that the fighters are. Global goals are actually different. A global goal is the main goal for the mission. If all the goals for a specific flight group are used up, you can make a global goal. It's really just for making extra goals that won't fit elsewhere.
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