Mobs are divided in seven categories, each following a set of rules determining maximum number of mobs in the player vicinity, whether they should spawn in difficulties above peaceful, spawn range, etc. Mob categories are specified when creating entries for mob additions, removals, and replacements to determine which rules the entry should follow.

It is important to note that beyond the rules specified in the Mob Spawn Group options, a particular mob may have its own specific spawning rules which may prevent it from spawning if added to a biome.

Hostile mobs including zombies, skeletons, endermen, etc.

Passive mobs including pigs, cows, bees, turtles, etc.

Mobs including exclusively bats.

Mobs including exclusively axolotls.

Mobs including exclusively glow squids.

Underwater mobs including squids and dolphins.

Non-mob entities including chest boats, activated TNT, etc. Does not naturally spawn.