Hi

You don't have to change the catagory and subcategory fields - keep them as strings.

You can build it in a client script or a business rule, depending on when you want the fields to change (real time or when the record is saved)

The client script or business rule will run when the category and subcategory fields are changed and will query the ABC table for the specific combination.

The record which matches, will contain the CI while you will then apply to the CI field on the incident.