When you have two dropdowns that relate to one another, you might want the field selection of the first to determine which fields appear in the second. You can do this with Field Dependencies.
To set up a field dependency:
- Go to System Settings > Objects and Fields.
- Select the object you want to create a field dependency for.
- From the Object Management list, select Field Dependencies. Click Add Dependency Mapping.
- Select your Parent Field and your Dependent Field.
- Select the dependent field values. To select more than one value, hold the Ctrl key while clicking on each value.
- Click Preview before you save to test your mappings.
- Click Save Mappings.