Purchase order is a commercial document issued by buyer to 
a seller indicating the type,quantities and agreed prices 
of products or services that the seller will provide to 
buyer.Gradually there are there types of purchase order.
1) Blanket purchase order-this is used for low cost item(eg-
stationary)are purchased frequently.the po is issued with 
the limit on purchase amount.Within that amount the buyer 
can keep ordering the material as frequently required.when 
the po limit is exhausted a fresh po is created.

2)standard purchase order-this are most commanly used 
purchase order.In this case the buyer has the detail about 
what he or she is buying like quantity,price,delivery 
schedule etc.
3)Planned purchase order-The buyer commits to buy from the 
supplier over a long term(e.g.raw materials for 
construction project like cement,steel,glass,bricks 
etc).the tentive delivary schedules are agreed upon along 
with estimates of qauntity and price of goods/services.this 
helps bring down the inventory that buyer may have to hold 
to avoid outages.this also enables JIT supplies.