Minecraft wiki entry for Spawn, associated text: There are two types of environmental spawns: cluster spawns and structure spawns. Structure spawns reproduce specific types of mobs at specific locations within certain naturally generated structures, such as nether fortresses, swamp huts, etc. Cluster spawns account for all other types of environmental spawns, including mobs that spawn individually (i.e. not in a herd of 2 or more). Both types of environmental spawns follow the same rules for spawn conditions and mob caps, except that structure spawns can exceed the monster population cap by 1 (see below).