BPA with Sourcing and ASL

 Hi Experts

I have few confusions, as follows:

1) Is it necessary to create ASL and sourcing rules with assignments so that when you create requisition or PO, the system indicates that you have a BPA? Main question is are sourcing rules and assignments necessary?

2) For the local BPA I created, I create requisition and the system indicates that BPA exists, and in the Source details tab it would show the BPA references.

However when I create a PO directly, it does indicate that a BPA exists but I dont see any details under the reference documents tab..why is that? is it standard functionality?