Rosette's character, owing to her more scholarly than magically taught background, has a large amount of "book knowledge" (theory) as to magic and the wider supernatural. However, she is intentionally written as lacking direct training in practical or applied magic, so that while she could explain how and why to cast a Fireball spell, she would rather likely struggle if she had to cast it directly as opposed to via scroll or other device. In short, she's talented in theory but unskilled in practical magic.