Well everone will be different of course.
We use categories to generate the suggested assignment and to report on Service Availability.
Also to distinguish between Service Requests & true "Incidents".
Each category has a relationship to a Service (from our catalogue) and a Support Group and is 3 levels.
Level 1 Level 2 Level 3 Service 1st Assign Group Corporate Apps Delphi Fault Payroll Business Apps Desktop Apps Adobe Reader Service Request Desktop Services Workstations Blackberry Fault Personal Productivity Helpdesk Printing Printer Fault Printing Workstations Security Password Reset Security & Accounts Helpdesk Telephony Corporate Fault Telephony Communications