It's a Type A visa for changing flights inside the international transit zone of a Schengen airport without entering the country. It's required only for certain nationalities and certain transit situations.
If your nationality needs a VTA and you're transiting through an Italian airport, you have to get one before your flight. Without it, you can be refused boarding.
Most travellers transiting through Schengen don't need a VTA. It applies only to specific nationalities or situations, so always check before assuming you need one.
Official source
Ministero degli Affari Esteri
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