The Clause Selection screen pulls double duty. It can be used not only to select individual clauses to build a document from scratch, but to select complete documents (i.e., 'templates') as well. 

'Clause' assembly occurs when you have, let's say, 100 clauses appearing in the left side of the screen. You might select 5, 10 or 50 of those clauses to make a final complete document. The goal of this kind of document assembly (or perhaps better, 'clause' assembly) is the creation of a 'perfect' draft of a document starting from a blank screen.

If the above is 'Clause' assembly, its sister is 'Document Assembly'. Document Assembly in this context is taking a collection of different documents and creating a 'package' of documents needed to complete a transaction. (Perhaps it should be called 'package assembly'.) When you assemble documents in this fashion, be sure to check out the 'Additional Assembly Options' menu located at the right of the Clause Selection screen. Among others, those options allow you to add 'page' or 'section' breaks between the documents you have selected, or to save (before processing the documents) each selected document in a new folder for a specific client. Read more about the latter option at this link.