Campaign scoring
From Halopedia, the Halo wiki
Meta-Games are included in the campaign for Halo 3. They are confirmed as a way to collect achievements. The Meta-Game is a Halo 3 campaign mode, where by playing well, you recieve points. Having skulls activated increases the amount of points that you gain, as well as accuracy and kill count. Meta-Games are a way to keep track of points in campaign mode. This is especially useful in cooperative gameplay to create a competitive game between players in co-op. Different enemies give different points as do headshots (and assumably any other kill that would be worth a medal). Betrayals will cause negative points so cooperative work will be necessary.