Copy crontrol from service contract to billing documents

Dear colleagues,

I need to do development to copy the PO number maintained in a contract to billing documents (BDR or invoice).

I think a copy control need to be created at first, but I didn't created any copy control in the past, can anyone help me?

1. Where and how to create a copy control from service contract to billing document.

2. When I created the copy control, where I can write code to enhance it?

Thanks very much!