What does the controlling field determine when field dependencies are being created?

The values that appear in the dependent field.

Field dependency means controlling a fields value based on the other field. Controlling Field is a type of picklist field that controls what values will become available on its designated dependent fields. It plays a crucial role in linking together two sets of picklists. Controlling Field is an important part of Field Dependencies, a feature that lets users filter picklist field values depending on another field’s contents. You can use any custom or standard picklist with less than 300 values, or any checkbox field within a single record as a controlling field.