A FusionAuth Application is simply something a user can log into. When you use the Login API, you will provide an
applicationId to indicate what resource you’re attempting to obtain authorization.
When you use one of the OAuth2 / OpenID Connect authorization grants you will provide a
client_id in some fashion. This client identifier will be unique to a single FusionAuth application which allows FusionAuth to to verify the User is registered for the requested Application and subsequently return the correct roles.