As mentioned in Chapter 1 (Essential Modeling Heuristics), the central feature of the environmental model is a specification of the events to which the system must respond. In the following sections we will further explain the concept of an external event and describe methods for formulating an external event list for a system.
An event in a general sense is a significant occurrence. An external event has three characteristics:
It occurs in the system’s environment.
It elicits a preplanned response from the system.
It occurs at a specific point in time. ...