There are certain fields whose values depend on another field's value. This can be defined as a parent-child relationship. For example, let's say there are two fields - Country and State - in your Tickets module. The values in the State field will depend on the country that you select from the Country field. Similarly, the values for the Sub-Department field will depend on the Department field that you choose. Mapping dependency fields establish this kind of relationship. You can define which values will appear in the Child field when a particular value is chosen in the Parent field. Note that this is applicable only for picklist fields.
Click the Setup icon ( ) in the top bar.
Click Layouts and Fieldsunder the Customization menu.
Click Field Dependencies under the Layouts and Fields sub-menu.
In the Field Dependencies page, click Add New Dependency in the upper-right corner of the page.
In the Add New Dependency page, do the following:
Select the desired Module and the Department (when more than one exists) from the drop-down menu.
Select the Parent Field from the drop-down menu.
Select the Child Field from the drop-down menu.
Click Next
Click each of your parent field values to select their corresponding child values. A checkmark indicates your selection of values for a parent field value.