Because incidents are used to log issues or things that are broken, the available categories on the incident form correspond with common issues. The Category designates the type of service impacted (not the specific service), and the Subcategory provides relevant common issues based on which category was selected (i.e. Applications > Launch Issue).
The Affected CI field should be used to tie an incident to a specific service.
Proper incident categorization ensures accurate reports and metrics, and will also help build a foundation for future Problem and Event Management tools in ServiceNow.
While Request tickets don't require categorization in ServiceNow, they are organized under categories and subcategories on the UT Self-Service Website. Categorization on the self-service website is configured on a particular service's Business Services page in ServiceNow.
Since Requests do not involve break/fix, the categorization scheme is not the same as Incident but is instead geared toward simple end-user navigation. The Request Category is generally the type of service (should match Incident categories), and the Subcategory is the specific service (i.e. Software and Applications > ServiceNow).
In some cases, a service can be tied to multiple Request Categories to make them more easily accessible to end users searching the self-service website.