Purchase Orders

Can someone give me a quick insight on how they use the PO's in the Project Manager module?  I am not understanding how the flow of this is supposed to work.  We need to issue a PO to one vendor but need to track them by phase (I think I have to issue one per phase) but also not quite understanding when you put the received information in etc.  I would assume we want to show as a committed cost when we place the order but the way they make it sound in the training videos is you would issue the PO when it is received?  

Any help/experience you have with PO's would be greatly appreciated.