About Auxiliary Equation-Based Nodes
The PDE Interfaces provide an environment for specifying a model in terms of equations. In many cases, however, you may only be interested in adding an equation term or a constraint to the PDE implemented by a physics interface.
The auxiliary equation-based nodes are found under the context submenus More>, Edges>, and Points>. To display these in the context menu, click the Show button () on the Model Builder toolbar and select Advanced Physics Options.
These are the available nodes (listed in alphabetical order):