I don't know if you figured this out already, but most of this can be answered by looking in gamestages.xml.
1/ and 2/ - These are used for wandering hordes. See the "WanderingHordes" spawner.
3/ - These are some of the groups used for sleeper volumes in POIs. See the "SleeperGSList" spawner, which is used by the "S_-Group_Generic_Zombie" group.
4/ - These are more groups used for sleeper volumes. See the "ZombieBadassOnlyHorde" spawner, which is used by the "S_-_Group_Zom_Badass_Only" group.
5/ - These groups are used to spawn entities that respond to a screamer. See the "ScoutGSList" spawner.
6/ - These are the groups used for blood moon hordes. See the "BloodMoonHorde" spawner.