If permission is required, what permission is it, where/how is it obtained, and what core onward-travel evidence is required?
The permission is an airport transit visa, examined and decided by consulates. Applicants must present documents on continuing the journey to the final destination after the planned airport transit and documents establishing their intention not to enter Member-State territory; the consulate verifies the travel document is not false, the coherence of origin, destination, itinerary and transit, and proof of onward travel.
See recorded sources