purchase order vs purchasing

purchase order

noun
  • A commercial document issued by a buyer to a seller, indicating the type, quantities and agreed prices for products or services the seller will provide to the buyer. 

purchasing

noun
  • An act or process of making a purchase. 

  • A part of an organization that manages such processes 

  • A complex, organized process in large organizations for obtaining goods that may include identification of requirements, approvals, supplier management, negotiation, receipt of goods, and payment. 

How often have the words purchase order and purchasing occurred in a corpus of books? (source: Google Ngram Viewer )