I know Jason 47 can explain it better than I can......but, headwriters do the bible, or "this is what is going to happen. Breakdown writers then break the story into scenes, depending on how long the story is to go on. And script writers do the dialogue.
i.e. The bible may say, surprise birthday party for Paige where Paige , The breakdown writers then break that down into several scenes, Decorating, guests arriving, Paige's arrival, the cake brought out, Paige's fury, etc. Dialogue is inserted into each scene, the meaningless chatter, the rage, anger, shouting, etc. etc.
Sounds like a good explanation to me!