What if it is both purchase of tangible assets and services thereof?
Generally Work order is of such a nature it includes material cost and Labour these materials are for some specific Purpose say Civil Work in a Factory and which is not a regular usage of materials meant for the factory so in such a case Work order is raised not a Purchase Order
Purchase Order is raised only for regular Raw Materials,Capital Spares,Consumable Stores and the Like
Generally work order or job order (sometimes job ticket or work ticket, as it often has some type of ticket attached) is an order received by an organization from a customer or client, or an order created internally within the organization. A work order may be for products or services.
And a Purchase Order is the official document for all purchasing activities. The terms and conditions of the purchase order give legal protection to the initiating department and to the company. This order also provides legal reference and backup in disputes over shipping, quality, damage, etc.