Delta Airlines facilitates their most valued users with the preferential advance seating arrangements. This service is termed as Preferred Seat Assignment("P" seats). Preferred Seating on Delta and the Connection® flights, depends on the accessibility.  One must purchase the ticket before the allotment of a Preferred Seat (P seat). Preferred Seating is not qualified for the customers who bought Basic Economy. 

Remember: Preferred seats might be reassigned to travelers with certain classes of disabled people.

Who Can Avail the Delta Airlines Preferred Seat?

Except basic economy fare holder, Delta Preferred seating is accessible anytime after reservations for:

The people booked full B, M and Y fares on Delta Airlines Flight.

Source: https://fistpal.com/blogs/18176/Eligibility-to-Seek-Delta-Airlines-Preferred-Seat