Demand Driven Acquisition (DDA, also called Patron Driven Acquisition or PDA) lets your patrons drive which items you purchase. With your vendor, you set up a collection of titles available for DDA, and then a purchase is triggered when a patron accesses that item.
Because you cannot know which items will be purchased ahead of time, you can create an order of the type DDA Plan, and then add items to this order as they are purchased. This then lets you receive and invoice items as per normal.
The workflow is:
Acquisitions terms explains the DDA Plan order type.
About DDA collections explains how DDA collections are maintained in WorldShare Collection Manager.
You can track when your vendor updates your KB collections by checking Metadata > Collection Manager > Activity History. See Collection Manager Activity History for details.