This mod adds new spawn eggs to the game. These spawn eggs can contain NBT data which will be used to modify the spawn egg’s properties/entity information. By default, the mod duplicates the vanilla spawn eggs and adds special mobs like Powered Creepers, Wither Skeletons, Villager Zombies, etc.
For map makers, you have a lot of choices. You can use the pre-defined spawn eggs by simply changing the item damage – no need to even add other NBT tags. However, if you want a custom mob, you can modify the NBT tags on ANY of the mod’s spawn eggs, although modifying the “Custom” egg (damage=0) is recommended.