booking vs purchase order

booking

noun
  • The act or process of writing something down in a book or books, e.g. in accounting. 

  • The issuing of a caution which is usually written down in a book, and results in a yellow card or (after two bookings) a red card, that is to say, the player is sent from the field of play. 

  • The process of photographing, fingerprinting and recording the identifying data of a suspect following arrest. 

  • The engagement of a performer for a particular performance. 

  • A reservation for a service, such as travel or hotel accommodation. 

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. 

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