Service Requests involve delivery of some sort of service like laptop, phone, tablet etc., to the requester. This type of requests can be completed only after a series of tasks are performed. For example, in case of a laptop delivery, the technician will check whether the requested asset is available (in store) and might have to seek for higher authority's approval before proceeding with the service delivery. Now, when the requested service is not in store, then service delivery process has to begin from scratch: that is, first the purchase order (PO) for the service request will be created and submitted to the concerned vendor; then the asset will be acquired and provided to the requester.
Since Service Requests are often associated with Purchase Orders (maybe new or existing), ServiceDesk Plus provides an option to associate Service Requests with Purchase Orders (and vice versa) from the request details page as well as from the purchase details page.
To associate service request with purchase order, do the following:

To view details of the purchase order (PO) associated with the Service Request, do the following:
Select request by clicking on the request subject
Click the number next to Associate Purchase Orders in the right panel of the request details page
Click the purchase order number to view it in print preview mode
Requesters can view the list of associated purchase orders but cannot access the PO details
In case the technician wants to delete the purchase orders associated with the service request once the purchase is completed, he/she can do so by following the steps given below:
Click the subject of the request to open it
Click the number of associated purchase orders from the right panel
Hover the mouse on the required purchase order and click Detach

Technicians of the Service Request will be informed about the Purchase Order's status - whether the PO related items have been received, partially received (or) whether the Purchase Order has been rejected by higher authority - through notifications
Any number of Purchase Orders can be associated with a Service Request