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mduell: He/she was making the point that if there is P9, there is likely at least J18 (i.e., if they are willing to sell 9 P seats, there is very likely more than 9 available for full-fare business class).
I don't know if this is true, but it seems reasonable to me.
No, it's not true that P9 means J>9. It means J>=9. A flight can be booked 41/50 and the inventory can be J9 ... P9. It means they are willing to sell 9 P fare tickets, and they are willing to sell 9 J fare tickets, but it does not mean they are willing to sell 9 P fare tickets and 9 J fare tickets.