When fields from multiple category sets are added to a report, the Report Builder must know how the fields are related to one another.
A report for claims entered in a specific date range may contain data from the following fields: Patient Name, Patient Date of Birth, Claim From Date, and Claim Amount. These fields are from both the patient category and the claim category so, the Report Builder must relate the claim category fields to the patient category fields, based on the patient account number.
In this case, the Report Builder automatically joins the two (e.g., patient and claim category) because there is only one possibility.