Except when the customer is a business located outside your own country but in the EU, in which case reverse charge applies, so you are not allowed to charge VA

Except when the customer is a business located outside your own country but in the EU, in which case reverse charge applies, so you are not allowed to charge VAT, but instead the buyer has to pay VAT in their country (well, "pay" as in "pay and immediately get it refunded" in most cases). Plus, if you are making more than some threshold in a particular country from sales to non-businesses, you might be required to pay VAT in that country, and charge your customers that country's rate. And it also depends on what you are actually selling. And I may well have confused some details, so don't rely on this as tax advice, other than the fact that it's somewhat complicated and difficult to get right.