The country which is decisive for taxation is the customer's tax country. If there is only one billing address, the country of the billing address determines the taxation and defines the tax country. If there is a VAT ID number different from the billing address country, the VAT ID number determines the taxation and defines the tax country. If a destination country different from the billing address is passed for a customer, the destination country determines the taxation and the tax country. If a VAT ID number and a different destination country is submitted, the VAT ID number determines the taxation and the tax country.
The tax country used is matched with the tax rules stored in billwerk and thus a customer with Germany as tax country will receive the tax rate configured for Germany. If the country of the customer is not explicitly defined as tax country, it will be checked if the country is located in the European Union. If this is the case, the configured tax rate will be used. If the country is outside the European Union, the tax rate for the "Rest of the World" will be used. You can find more information about the tax policies here.
If you want to set or change the customer's tax country via REST API, you need to populate the "DestinationCountry" value in the Customer object. For the country codes, the two-digit ISO 3166-1 code is used. You can find a list here: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2