Delta Airlines tricks you. Despite what they say or show you online there is actually no seat reservation when you book through them on the Alitalia shared flights.
Alitalia will know nothing about what Delta is telling you when you get to the airport.The flight number and seats will not be recognized at the Alitalia terminal.
Delta will give you the company line that they are in a partnership and can't answer for the other airline. Delta misleads you to think it is a Delta managed flight and show seats assignments online when all they have done is issued you a ticket on behalf of Alitalia.
Review about: Delta Airlines Flight.