A scenario has a few advanced running settings. Here they are
Under single threading mode the scenario processes only one object at a time. It means that the other objects (if there are any) are waiting in queue at Start step till the object under processing quit the scenario.
If the option is ON, scenario will create objects to SystemExceptionsystem data structure when something goes wrong.
That is a way to avoid collisions writing to a single field on an object. The option is OFF by default for the processing speed purposes.