If you compare this definition with that of a command from earlier in the chapter, they follow the same general premise, but the differences are very important.
An event is a message that is published from a single logical sender, and is processed by zero or more logical receivers.
A command can be sent by any number of different senders. An event is only published by one logical sender. Sending a command sends one copy of a message to one receiver, and that receiver is the only entity that can process that command. In contrast, when an event is published, a copy of that message may be sent to dozens or even hundreds of subscribers, or maybe none if there are no subscribers.
This has even broader implications. While a command is an order ...