Catalyst Monsters are required for the Reaction Summon of Reactor Monsters. When the product Level of 2 or more monsters Reactor Monster Product, by sending them to the Graveyard, you can Summon that Reactor Monsters.
Catalyst Monsters are a new secondary Type for Normal Monsters. They can also be another Type of Effect Monsters, like Union and Spirit. Catalyst Monsters can be either Effect Monsters or Normal Monsters or even Reactor Monsters