Thanks, that is what I suspected.  Do you know if there is any place where there is a general discussion of the relationship between projects, drawings, layouts and blocks.

As far as I can tell a project is a list of layouts which may be in more than one DWG.  A DWG consists of a model and a set of layouts.  Blocks seem to belong to drawings.  There must be a way to create a custom block libraray but I have not learned it.