Locked Licences
The Locked Licences page allows you to pre-allocate a number of licences to users or workstations. This means they are guaranteed to receive these licences if they request them. For instance, you might pre-allocate licences to all of your designer workstations. Whenever a designer connects and requests a licence, they would be granted their allocated licences. The Allow locked licences... option means other users or workstations could use the licences until the designer requested the licences. When the designer connected, the licences are automatically granted to the designer and the previous user would need to request another available licence to continue using Impact.
You can also hide a licence from a user or workstation; this just prevents the licence from being displayed in their available licences list.