Thread:707Mithrandir/@comment-28905095-20170317182622/@comment-26335978-20170317184431

It is similar to spawning the mob normally, but instead of directly spawning the mob, it spawns numerous mobs over time. Also, it is more just modifying the monster spawner, rather than being a separate block inside the spawner.

Placeable is a broad category so that might still fit within it.