Final answer:
The concept of arrival rate in the electronic message center refers to the rate at which messages are received. An average of 9 messages signifies the rate at which messages are received.
Step-by-step explanation:
The concept of arrival rate is used to describe the rate at which events or messages arrive in a system. In the context of the electronic message center, arrival rate refers to the rate at which messages are received.
When it is mentioned that there is an average of 9 messages, it signifies the average number of messages received per unit of time. In this case, the rate at which messages are received is 9 messages per unit of time, whether it is per hour, per day, or any other specified time frame.