The Notes Document container allows you to use documents in your composites. This is a new feature in Notes 8.5.1 and an article showing one of the many use cases for this container was posted on the CA wiki. The concept of the document container is to easy get data into and out of Notes fields – whether it is in read only mode or edit mode and share that data with other components. This article is the first, and most basic, use cases but also shows how you can manage the life cycle of the document component.
Referenced article is here.