If you plan to constrain a specific surface region, your model needs to include the contour that defines the region. Before you attempt this type of constraint, see
divide surface features, which describes how to create a region from a datum curve.
For assemblies, you cannot constrain geometry that
Creo Simulate merges during a run, such as mated surfaces. You also cannot constrain geometry associated with merged geometry, such as points associated with mated surfaces. If you cannot avoid placing constraints on these geometric entities, you need to work with regions. For more information, see
Assembly Considerations.