Field help (ACATSEARCH) 

A category acts as a filter on the search results.

For instance, if you define a category "SALESREP", you can assign it to the Sales rep field in a customer record. A Sales rep field is also available on the sales order and sales invoices. When you search for "MARTIN", if "MARTIN" is a sales rep present on 20 customer records, 45 sales orders, and 35 sales invoices, selecting only the sales rep category will restrict the search results to these elements, while "MARTIN" might also be present on thousands of other entities.

This category is defined on the miscellaneous table n°16.