PN stands for "part number" - 7287270000 is the part number for the box itself. A lot of Transformers packaging have a PN on them somewhere.
Just grabbing one that's handy...yep. Bumblebee Battle Suit (from Prime) has PN 7247430000 on it, right after "MADE IN VIETNAM."
You'll find this number on the box, a different one on the insert, and so on. Defensis Prime used to be the master of little details like this - here's a snippet of an old conversation concerning PNs:
And when I said 'every part', I meant it. For example...SDCC 2008 Nemesis Prime.
Cardboard Box: PN 6806430000
Cardboard Insert: PN 6800700000
Instructions Sheet: PN 6812720000
Frankly, it gets a bit problematic at points. Beast Wars carded figures...that bubble sticker probably had a part number, and we will likely never know it, 'cus it was probably printed on the frame.
MIKE
engledogg