Constraints in Primavera P6

Some planners find it hard to differentiate between Lag and Constraints in Primavera P6 and tend to use them interchangeably because, under certain very limited circumstances, they produce similar results apparently. These are, in fact, entirely different functions. Lag is used to delay (or advance in case of negative Lag) an activity by certain duration units due to some purely technical/logical reasons while Constraints are used, generally in addition to logical relationships, to depict ground realities that cannot be otherwise built through logical relationships (Dependencies) only. Consider this example: we plan to prepare a concrete foundation and place certain equipment [...]