The User Settable value is either 'true' or 'false'. It indicates if a non-help desk admin is allowed to set the category value on a ticket.
Many of our customers have a simplified view of Categories that they present to their end users. Hardware/Software/Email/Network/Printers for instance.
But internal to the help desk admins, there are various levels of Network problems and Software Problems. This allows you to have a complicated set of Categories for internal use, while keeping it simple for your end users.