A feature that puts some sort of constraint on a dependent variable (a Dirichlet boundary condition, for example) needs a Constraint node (). A constraint forces an expression to be zero, using the expression together with a constraint-force expression. In most cases, the constraint force can be generated from the constraint expression, but the constraint node also provides the possibility to customize the constraint force. You can add an Excluding Selection subnode to exclude selections from the selections that the parent Constraint node already has.

Constraints can also be formulated as a Weak Form Equation using an extra degree of freedom. This technique is called weak constraints, and you can add a Weak Constraint node () to add such a weak constraint.