If your store support multiple countries, there is a place where you may manage those. 

As you may notice, there is possible to adding limitations, like restrict specific countries to be used as a billing or shipping. Also may add notification if country is a subject to VAT, so when customer will apply country with that rule, a VAT will be added (if configured). 

Each country have own states. Here you may add, if will open country details page. From the main page, you may use export/import as well. 

State field is required if states are enabled in your store. Need to add states, otherwise no-one will be able to place an order.

Limitation in action

With restrictions need to be careful, as may occur an issue. Assume, a citizen of Antarctica will not be allowed for billing. Note, allowed for shipping.

When such a customer will try to complete an order, will see an error.