If your business is interacting with other businesses within the EU, you are required to provide a so-called ‘VAT-Number’.
Every VAT-Number consists of a duble-digit country-code followed by two to twelve further digits. VAT-Numbers in Austria for example start with the countrycode ‘AT’ followed by ‘U’ and further eitght digits (e.g. ATU12345678).
details can be found here: https://www.bmf.gv.at/themen/steuern/fuer-unternehmen/umsatzsteuer.html