When you create a user, the system stores data about the user in the database. After the user logs in to the system, the system assigns a license to the user. You can have two types of users: authorized users and floating users. Authorized users can always log in, but always consume a license. Floating users only consume a license when they are logged in, but may not be able to log in if all the floating licenses are in use.
If you create more users than you have licenses, the new users cannot log in until you purchase additional licenses or use the root user account to remove licenses from some users (or delete some users from the system).
A user can only be logged in once. If you are logged in on one machine, and you log in from another machine under the same name, the original session is invalidated.