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 the Admin.
Admin (or Administrator) is a person who has access to the app and can manage signatures, marketing planner, and payment methods.
People flagged as "Marketing" have access to the marketing planner, but are unable to make any payments, change payment methods, or alter any User info.
Root has the same rights as Admin and, on top of that, can assign roles. The Superadmin in your Google Workspace account is automatically assigned the Root admin access. However, this can be changed in Settings.
Light Admin can access most parts of the application except Billing, Billing Information, Subscription, Credit Automation, and Automation Features.
The Signature Designer role gives a user access only to Signature Templates and the possibility to preview a signature for any User.