The Users Window shows every client who has connected to the KeyServer at some point. Users can be used as parameters to reports, and can be associated with User based Policies. To see more details, double click on a user to get the User Details Window.
The User Details Window has a small header area and three panes - Information, Policies, and Notes.
The Information pane shows where and when this user last logged in, as well as what folder has been assigned in KeyConfigure. There is also a text field labeled "External ID" which allows you to associate another id with each user, presumably to link to another system. Finally, the Tags field allows you to associate arbitrary strings with each User. For more, read the Tags documentation.
The Policies pane shows the list of Policies currently assigned to this user. For more details on user based license metrics, refer to the Policy Details documentation. Note that this list of policies does NOT include all policies this user has used - only those policies with user based metrics that are currently allocated to this user. Policies can also be assigned by computer as opposed to user. If you want to determine ALL policies that a user has used, right-click the user and run the Usage (USER x plcy) report, choosing an appropriate time range.
The Notes pane contains a single item, which is a free-form text field. You can use it for any information you want. It can only be seen here in the KeyConfigure interface.