On Delta Air reservation portal you can select your favourite seat by three clicks, In some cases system automatically assign a seat as per the history of your previous selections. If your preferred seat is not available, you will be assigned a seat after check-in on the departure date. Your pre-assigned seat may change in some cases as Delta Air prefers seat assignment of the passengers with certain disabilities. Your cooperation is required in such special cases. Passengers who opted for basic economy fare can't take a seat selection before check-in.