Help with official box score (PDF) for Nebraska game.

Hi, sorry to bother everyone but hoped someone could help. I get the official box scores as pdf files from the UM Goblue.com athletic site. 

There is a play-by-play breakdown, drive by drive breakdown and other useful info. Evidently a program called The Automated Scorebook generates it.

It is pretty clear and largely self explanatory. One question I hoped someone else here might know relates the the small two letter codes - here is a clipped view. There are "P1", "R1",..., from what seems clear P means a pass, R means a run and X means a penalty, but I'm puzzled by the numbers. At first it looked like P1 mean the first pass but the numbers don't fit into the first pass of a series, first pass of the quarter or first pass overall. ? any thoughts?

Here is the link for the pdf  file: https://mgoblue.com/documents/2018/9/22//20180922_fbl_nebraska.pdf

Thanks and Go Blue! I hope it is just something simple and obvious that I am just overlooking.

Was at the game and really it was a terrific total performance.