Each user belongs to only one user-type. From this option you can add or customize the user-types according to your needs.
Each user-type may select characteristics from a predefined set of roles and with different permissions for each role (1). The 3 available roles are: the Admin, the Instructor and the Learner (2).
When you first create your domain we create 4 default user-types. A SuperAdmin that cannot be changed and is assigned to the initial account owner, an Admin-type that has all available roles, an Instructor-type that has the role of Instructor and a Learner type. By default an Instructor shares the Learner characteristics so he can see what the course looks-like for the learner.