Blanket purchase orders allow you to raise an order that specifies items and price, but does not necessarily specify the job on which the items will be used. The job and costing details are entered with the delivery ticket (if used) or during invoice matching.
Features of blanket purchase orders:
Allow you to specify items and prices.
Optionally allow you to specify a date range.
May or may not require delivery tickets.
Do not create a committed cost.
There are two types of blanket purchase order: Type A does not allow you to enter the job; Type B requires you to enter the job, but not WBS details.