Each location usually has Primary, Secondary (possibly Tertiary as well) and Rare Monster groups, and the game will load up a sequence at random based on which sets are available (usually through story progression). Depending on the location's map, the following can happen:
- one set of the Primary Regular monsters will spawn
- a Secondary Regular will spawn or a Rare monster will spawn
- one set of Primary Regular
- a Secondary Regular
- a Tertiary Regular or Rare
- one set of Primary Regular monsters will spawn as there are no Secondary Regulars
- one set of Primary Regulars
- a Secondary Regular as there are no Rares left to spawn
- very rarely, nothing will spawn (Darken Forest - Abyssal Forest is one such example)
There may be other patterns that haven't been observed. It's more likely that if there isn't a Secondary monster present that a Rare has spawned.
Note - Map does not mean location. Location = 1 or more maps.