It is a common scenario where there is a test (or QA) database, and a production database, and in the interests of theft or fraud mitigation, you would want the reports (such as invoices, delivery dockets, pack slips or credit notes) to be easily identifiable as coming from a test environment.
The image to use as a watermark is attached to the plugin as a document - it can be any image Crystal Reports recognises - jpg is what I used as png gave me odd results with the transparency.
Sample output (from a sales order invoice):
