ATC issues popup clearances to pilots without them having
filed any flight plan beforehand.

Is the clearance issued before ATC has the information it requests of
the pilot (which would then constitute the flight plan)?

Jose
--
There are two kinds of people in the world. Those that just want to
know what button to push, and those that want to know what happens when
they push the button.
for Email, make the obvious change in the address.