A few airlines use P for Premium first class. AA uses it for their "real" first class on 3-class domestic flights (on two class flights they call the front cabin "first" but it's really business and their international codeshares sell it that way to avoid a revolt among the natives.) Singapore used to use P for full-fare first, and F for discounted first, but I think when they joined star they went to the more common F/A for full fare/discount.