Several months ago my coworker asked me for seating advice. Both she (one reservation for family of 5) and her sister (second reservation for two) had regular economy booked. Reservation one had lots of options for seating in the back half of the economy with the front half all Xs, reservation two had a handful of options in the front half of economy with the back half all Xs. No overlap between the two. She called in to ask if they could all get seats together and the agent mentioned delta does this to give families ability to sit together. First I ever saw anything like this. It was a 767 DTW to MCO.