It's... tied into destiny, somehow. So, sure, on the surface, Dash's mark might be implying her flight speed and her ability to pull a rainboom out of her rump like it wasn't any business, but... underneath that, it's keeping her on her path. Reminding her of who she is. It's... part of her, you know?

[Shame she hasn't had a chance to talk too much with you, Minato, or she'd liken it to a person's arcana.]