Scalar uses a registration key to restrict open registrations. This came about in 2017 when the developers discovered a bug that allows anyone to register for a public facing scalar.usc.edu/works install. When the key is generated, the user is required to add it to the user registration form when signing up for an account.
Reclaim Hosting’s installer for Scalar creates this automatically so you do not need to generate your own if you’re hosting with Reclaim. The key is added to your account and users are not required to add this to their registration form when signing up.
You can read more about the Registration Key here: Restricting new-user registrations on a custom Scalar install