market order vs purchase order

market order and purchase order both are nouns.

Word	Noun	Adjective	Verb	Adverb
market order	Yes	No	No	No
purchase order	Yes	No	No	No

As nouns, purchase order is a hypernym of market order; that is, purchase order is a word with a broader meaning than market order:

market order: an order to a broker to sell or buy stocks or commodities at the prevailing market price
purchase order: a commercial document used to request someone to supply something in return for payment and providing specifications and quantities

Other hypernyms of market order include order.

market order (noun)	purchase order (noun)
an order to a broker to sell or buy stocks or commodities at the prevailing market price	a commercial document used to request someone to supply something in return for payment and providing specifications and quantities