Domestically, P on DL generally means a discounted seat in the First Class cabin. They are usually capacity controlled, meaning a limited number of seats are available at the P price. They are often, but not always, non-refundable and have change penalties that non discounted or full First Class fares do not have. They are a great way to get a discounted First Class fare that, on occasion, is cheaper than a non-discounted full Y fare. But the caveat is the non-refundable status on most, but not all P fares. Check your fare rules carefully. As for seating, you may choose any available seat in the First Class cabin. Aircraft type will determine the seating type.