You can show any checkout field only when another field is filled with a specific value.
In order to achieve that please go to the checkout customizer: Products > Settings > Shopping Cart and click the edit checkout fields button. The checkout customizer will open up.
Each checkout field has rules section on the bottom of field edit box.
The first field is a drop-down with all currently available field in the checkout. The second field is where you should set a value of the selected field. After that the field will show up only when the user fills the selected field with the value specified in the rule.
Usage Example: we want to show the State drop-down only when the user selects USA as a country. The steps:
- Click on State field to open the edit box
- In the rules section select Country with value United States
After that the State field will show up only when the user selects United States as a country.