Calculator Question

Hi all, Random calculator question that has been bugging me. In the AMORT worksheet on the TI BA II+ calculator, what is the exact meaning of the P1 and P2 values? Say you calculate the payment on a 3 year, 2% monthly interest, 20,000 loan. Payment comes out to be $784.66. You go to the AMORT worksheet, and put in P1 = 12 P2 = 12 and get the stats on the 12th payment (PRN = 478 INT = 306). I’m ok up to this point. But what can you do with different P1 and P2 values?