Mail order and Purchase order

Mail order noun - A purchase negotiated by mail.

Purchase order and mail order are semantically related. In some cases you can use "Purchase order" instead a noun phrase "Mail order".

Purchase order noun - A commercial document used to request someone to supply something in return for payment and providing specifications and quantities.

Mail order and purchase order are semantically related. Sometimes you can replace phrase "Mail order" with "Purchase order", this phrases are similar.