Add a Global Constraint node () under Global Definitions > Equation Contributions to add a single constraint to the overall system of equations. The constraint is always enforced by elimination.

The Settings window contains the following sections:

Enter a globally valid Constraint expression. By default the constraint is enforced using a Symmetric reaction term such that the symmetry of an already symmetric system of equations is preserved. If some other form of Reaction terms is desired, select User defined and enter a custom Constraint force expression.