Reference Site for Purchasing Data

I am studying PA of retail 310 about reference site :refsite

ex:

You can maintain vendor and site data, purchasing info records and purchasing

conditions for a group of sites. Use the reference sites for the purchasing to do

this. Such a reference site represents all sites that are assigned to it. The data

is only stored at reference site level and is always used if there is no specific

corresponding data at store level.

As an extension of the supply source determination, the vendor and site data can

also be used to limit vendors to only the assigned sites. If a vendor is assigned to a

reference site, this vendor supplies all sites that are assigned to the reference site.

1 step: i have creat a refsite under a purchasing org,3101

2 step: creat a info for reference site  3101;

3.step: creat a po for 9005,but there is no price from the info condation reference 3101.

and now i want to know how to ues the  Reference Site for Purchasing Data???

who can help me?

Thanks