Just Another Spawner

Just Another Spawner (JAS) is a Minecraft Mod aimed at providing an alternative entity spawning system for Minecraft.

JAS adds an entity spawning system that runs in parallel to the vanilla system. By design, it fundamentally works the same as its vanilla counterpart, but allows for greater customization by exposing and consolidating many of the properties that are involved. Since it runs in parallel, by default, JAS will disable the vanilla entity system by emptying all biome spawnlists. It will import settings from entities that are declared in minecraft, including those added by mods, and set them to spawn within JAS spawner.

Primary source of Information / answers to questions is the  JAS Wiki or the MCF Thread.