If you sell to Austria, you must obtain an Austrian VAT number when the annual delivery threshold is reached!
The delivery thresholds themselves are determined by each member state and if they are exceeded, local registrations become indispensable.
With a delivery threshold of 35,000 €, this means that if, for example, you are registered in Germany and within one year you sell goods with this value to Austria via distance selling, the registration in Germany is no longer sufficient and a further registration in Austria is required. Registrations always mean that regular returns and filings have to be submitted.