Each license includes a certain number of admin users (or staff accounts). All licenses also include the ability to create unlimited KB Members (or client accounts).
Admin Users(one may also call them Writers, Editors, Authors, Superusers, or Admins) can log in to the front-end (public area) and the back-end (admin control panel). Their ability to access, create and edit articles, categories, and use KB functions depends on their admin user level.
KB Members(one may also call them Client Users, Special Users) are read-only users who can login to the front-end only. They cannot create or edit content. They can access certain restricted content under private categories. Their access to the restricted content depends on the user group that they are a part of. KB members can modify their profile, change log in password, and save a list of favorite articles. So, the idea of giving someone a 'KB Member' account is to provide him with ability to see something that other public users cannot see.