"Global rewards" are typically whatever the "goal" of a CG was. If the "goal" was "build a new space station", then the global reward is the new space station being built; the global rewards for additional tiers might be additional features installed in the new space station. These rewards are "global" because they affect everyone in the game, whether they participated in the CG or not; a "new space station" is accessible by everyone, not just those who took part in the CG.
Not all CGs have global rewards. Bounty-hunting CGs typically have no global rewards, as they are typically run alongside another CG (like a Trade CG) which does have global rewards.