Campaign models are usually hired for defined assets
A conventional campaign booking is tied to a production and a defined set of images or video. The relationship may be commercially significant, but the model is primarily talent inside a specific creative system.
Usage, territory and duration describe how far that production can travel after the shoot.
Ambassadorship extends into identity
A brand ambassador may appear repeatedly, attend events, post personally or become a recognisable public face of the brand. That closer association can increase both value and conflicts with future competitor work.
The best structure matches the real marketing plan instead of using a prestigious label to imply rights that were never clearly defined.