A system administrator can configure the license server to only allow specific users to request a license. This is available only for named-user licenses (not floating licenses) and is managed using an Allowed Users List.
In the license server management page, there is a link to the Allowed users list under Management Tasks. Also, in the Current Allocated Licenses page, there is a Show allowed users button. Both of them direct you to a page where you can see the Allowed Users List (if one has already been configured), along with instructions for configuring one. The list updates automatically between license requests every 30 seconds if the file is changed.