Is there any relationship between an ER Diagram that is created as a node in the Model Explorer and an ER Diagram that is created under a Subject Area, even if they both have the same name?
In erwin DM, every object is a part of a model. When you add an ER Diagram at the model level, that simply means that the objects in the model (basically all objects) can be added to that Diagram. Just because an object exists in a model, and the model has a diagram, that does not mean that the object is automatically added to that diagram also. It only means that the object can be added to that diagram. Once the object is added to that diagram, then it displays in that diagram.
With regards to Subject Areas (SAs), you can add one or more diagrams under SA, but that still does not mean that the objects in the SA will be included in those diagrams. The objects first need to be added from the model to the SA, and then only the objects that were added to a SA can now be added to a diagram under that SA. The following is an example of that:
Model has 5 tables - A, B, C, D, E.
Diagram under Model -- all 5 of these tables may be added to a diagram under the model.
SA under Model -- if only A, B, and C are added to a SA, then only those three tables can be added to a diagram under the SA. The diagram under a model is independent of a diagram under the SA - think of them as two separate diagrams within a model.