Sets provide a flexible (perhaps temporary) way of grouping your sources and nodes—for example, you might create a set for the documents, memos and nodes that belong to a specific team member.
Items in a set are references or 'shortcuts' to the original files—you do not physically move items into a set. This means you can delete an item from a set without removing it from your project.
A set can include any number of sources or nodes and a source or node can belong in multiple sets.