A dependent lookup is a relationship field with a lookup filter that references fields on the source object.
A dependent lookup is a relationship field with a lookup filter that references fields on the source object. For example, you can configure the case Contact field to only show contacts associated with the account selected in the case Account Name field.
When a user changes the value of a referenced field on the source object, Salesforce immediately verifies that the value in the dependent lookup still meets the lookup filter criteria. If the value doesn't meet the criteria, an error message is displayed and users can't save the record until the value is valid.
If the referenced field on the source object is a lookup, master-detail, or hierarchy field, users can't change its value by typing. Instead, users must click the lookup icon and select a value in the lookup search dialog.