User blog comment:Damac1214/Black Ops II Zombies Reveal Released/@comment-124.189.181.177-20120927071245/@comment-5472968-20120927100516

I'm guessing that it is for the campaign (Tranzit) because of the map that they show intermittently when they travel to different places on the map. Every area has a different kind of icon, so maybe each area that the bus takes you is a different level in the campaign that you have to complete an objective on, possibly something similar to actually making an Assault Door.