@ resposta do blackbird57 está correta, mas se refere a um site de terceiros em vez de relevantes FAQ no site da União Europeia Comissão:
An airport transit visa (ATV) authorises you to pass through the international transit zone at an airport located on the territory of the Schengen State and to await a connecting flight to a non-Schengen country. The ATV does not allow you to enter the Schengen territory (to stay at a hotel or to take an onwards flight to another Schengen State, for example).
isso é cristalino.
Eu estava lutando para encontrar um bom documento para mostrar que Tipo A significa Visto de Trânsito para o Aeroporto, mas eu o encontrei no Regulamento propriamente dito:
‘TYPE OF VISA’ heading:
In order to facilitate matters for the control authorities, this heading specifies the type of visa using the letters A, C and D as follows:
A: airport transit visa (as defined in Article 2(5) of this Regulation)