Primavera P6 supports 2 types of constraints; hard constraints and soft constraints.
In Primavera P6, “Mandatory” constraints are hard constraints. Hard constraints stand out because they can break relationships. An activity that has a Mandatory Start or Finish date becomes fixed to that date. Relationships to that activity are ignored – the activity will not move even if its predecessors push it out.
All of P6’s other constraints are considered soft constraints. A soft constraint is one that P6 will try to adhere to, but relationships take priority, and may result in the constraint not being met.