I dunno if there's ana rticle or anything that explains, but the way it works as I understand is as follows. The rewards for each bracket i.e. Top X% are determined by the current Tier that the CG has reached, as determined by static thresholds of tons delivered/bounties cashed in/whatever the goal demands.
Those rewards are, as you'll see, outlined on the Bulletin Board mision for the CG and in the reference in your Transactions tab. Your position in the bracket determines which of the rewards you get, in terms of the cash sum. So if you're in the Top 15% and the reward for Top 15% reads 2,500,000 credits + global rewards then you'll get that much cash plus the Global rewards.
Global Rewards are, again, dependent on teh current Tier that you'll see to the right hand side and could be as simple as 'Stage X completed;' which is a nebulous concept that FD use to determine how successful the goal was when writing up the results for GalNet, but it might be something more substantial such as '10% discount on module X for 14 days', all participants of the CG get to benefit from such rewards, hence their global nature.
Does that make sense? I' caffeine deprived this morning so I may be talking nonsense, but if you have any questions I'll try and clarify.