The term opt-out is not an official term, it is a term used by hellotax to define the intention to pay VAT in the destination country and to opt out from paying VAT in the origin country. In the image below, the Czech Republic is the origin country and the countries to opt-out are the destination countries. Read more about this.

  

So when you choose a country in the above menu, it means you will pay VAT in that country.