Basic Clarification on Formula and Average pricing

Hi Experts,

I have following clarification on F&A pricing in Purchasing.

1. We have provisional and Fianl prices. At the time of PO system will look for Provisional and then Final or first Final if no conditions then Provisional?

2. Why we have both Final and Provisional and when it will be used in the system? at the time of GR or Invoice?

3.Is there any link to get more knowledge on F & A?

Kindly help  me .

I guess Oily and Oily can help me on this.

Thanks in Advance

Savi Manickss.