"Global rewards" are in effect the Goal of the CG. They are going to be made available to everyone once the CG is over, not just the people who participated in the CG.
So the decals are not "global rewards", as they are only available to participants.
If the CG is the construction of a new megaship or space station, then the "global rewards" of that CG are the addition of a new megaship or space station to the game, which anyone will be able to dock at once it is built. If the Global rewards are new weapon types, then anyone will be able to obtain those weapons.
It should be noted that not all CGs have "global rewards", but the "global rewards" text is always present, whether there actually are any global rewards or not. Example: there have been "let's have a party" Trade CGs in the past, where the only rewards were the credits. No other global rewards were available.