What is a Blanket Purchase Order?

Blanket Purchase Order is an agreement that includes mostly 

i) Description of the items needed

ii) Unite price

iii) Contractual provisions

Blanket purchase order number is only set up one time, however it allows multiple releases against it at different times throughout a set time period.

A blanket order is used when multiple purchases are made of the same item or when items quantities are undetermined until usage is established.