A purchase order usually contains a list of services products, often including specifications and reference or part numbers of the items to be purchased

I am not sure what the question is but speaking from experience as one who has written dozens if not 100's of purchase orders, I know that to be successful in acquiring the product or services with the correct specifications and the correct amount they need to be accurate. Yes, the specifications must be clearly spelled out and to be able to do this takes careful research with the manufacturer or contractor so that the specs meet the intended needs of the purchaser. Also, the price can usually be negotiated with the supplier for example by explaining that one is a regular customer and frequently puts in significantly large orders or uses the same contractor for frequent contracts which are often significantly large.