Several users can work with a single database at the same time.
The database keeps a record of users who successfully logged into it, and suggests the username from of the user list when accessing it next time. The database assigns usernames to particular computers. This feature is very convenient in case the database is on the network and many operators work with it from different workstations.
You may want to monitor active sessions with the help of the "User monitor" form. It shows users who are working with the database and enables the system administrator to ask users to leave the database or to force them out.