The user is the person whose signature is managed in the app. Users don’t have permission to change their signatures in the app directly, but they can use a form to request contact info changes that must be approved by Admin.
Admin (or Administrator) is a person that has access to the app and can manage signatures, marketing planner, and payment methods.
People flagged as "Marketing" have access to marketing planner, but are unable to make any payments, change payment methods or alter any User info.
Root has got the same rights as Admin and on top of that can assign roles. The Superadmin in your G Suite account is automatically assigned the Root admin access. However, this can be changed in Settings.
The Signature Designer role gives a user access only to Signature Templates and the possibility to preview a signature for any User.