Copying Documents from the Product Structure 
Use
When you are editing a recipient list, you can use a
product structure to find documents that are to be distributed and then add them to the recipient list. You can copy documents from either display in the product structure; both list display and control display.
See:
Settings for the Product StructurePrerequisites
You must create a recipient list or change an existing one (Recipient List: Basic Data screen).
Procedure
You can copy documents from the product structure into the recipient list as follows:
Copying Documents from List Display
Copying Documents using Drag and Drop