Both issues' and copies' purpose is to output multiple copies of the reports. Copies are "real” hardware copies, meaning that the printer is assigned to create multiple copies of the output. Of course all copies are identical and will be created with the same printer settings.
If the output is to have different properties (e.g. Original from tray 1, copy from tray 2) or a "Copy” watermark is to be output, issues are the way to go. The property "Number of Issues” in the Designer has to be set to a value greater than one. Then the function "IssueIndex” is available for all regions, so that a region with the condition "IssueIndex()==1” (Original) and another with the condition "IssueIndex()==2” (Copy) can be created.
The objects in the Designer get a new property "Display Condition for Issue Print” with which the printing of a watermark can be realized in a similar way.
You can see a demonstration e.g. in the List & Label Sample Application (in the start menu's root level) under Design > Invoice > Invoice with issue print.