The IDEF0 model is a grouping mechanism for NOV-5 diagrams. In the model, you specify properties such as model viewpoint, purpose, and description.
In the model you build an NOV-5 context diagram, and then decompose that diagram with multiple child NOV-5 constraint diagrams.
Context diagrams
Every NOV-5 context (or A-0) diagram must be associated with a model. You must define a model name for the diagram when you create it. This rule complies with the IDEF0 specification.
Constraint diagrams
To provide flexibility in modeling, you are not forced to define a model for non-context NOV-5 diagrams. You can add one later.
You can create the model definition as you create an NOV-5 Context diagram, or you can create the model definition independently, and then specify it when you create the NOV-5 Context diagram.