Business events are classified by their trigger. An external event is initiated by an actor outside the business system, such as a customer. A time-based (or temporal) event is triggered by the passage of time.

1. Shop sales assistants ordering for a customer: The ultimate trigger is the customer's request. Since the customer is external to the organization, this is an External Event.

2. Customers ordering through the website: The customer, an external actor, directly initiates this event. This is also an External Event.

3. Sending a monthly catalogue: This event is explicitly scheduled to occur "monthly," making it a Time-based Event.

Therefore, the correct classification is two external events and one time-based event.

A. This option incorrectly classifies one of the customer-initiated orders as internal. The trigger for both order types originates from the external customer, not an internal state change.

C. This option incorrectly classifies both customer-initiated orders as internal. Events triggered by customers are external to the business system by definition.

D. This option incorrectly classifies the time-based event (monthly catalogue) as an external one. A scheduled, recurring action is a distinct time-based category.

1. Paul, D., & Cadle, J. (2020). Business Analysis (4th ed.). BCS, The Chartered Institute for IT. In Chapter 7, "Modelling Business Processes," business events are defined as triggers for processes. The text distinguishes between external triggers (e.g., a customer placing an order) and time-based triggers (e.g., the end of the month). This supports classifying both customer orders as external and the catalogue as time-based.

2. Laguna, M., & Marklund, J. (2013). Business Process Modeling, Simulation and Design (2nd ed.). CRC Press. Chapter 2, "Process Identification and Discovery," Section 2.2.1, defines event types: "An external event is an event that happens in the environment of the process... A time event is an event that is activated by time." This framework classifies customer actions as external and scheduled mailings as time events. (ISBN: 978-1466507789).