Channels are how messages are sent and received. Clients that subscribe to a channel will receive messages that are published to it.
Channels are lightweight and flexible. They exist merely by using them. Just publish or subscribe to a channel and nothing more. You can use as many channels as your app requires. There is no additional charge for the number of channels you use. Nevertheless, you should use them in a way that makes the most sense for your use case.
What you use channels for is completely defined by your application requirements. For example, a channel might be used for 1-to-1 private chat, many-to-many group chat, 1-to-many broadcasting, or many-to-1 fan-in use cases. A channel that is being used for private chat can suddenly be used for group chat. In other words, there are no predefined properties that need to be configured with channels. It is up to you to allow more users to start subscribing and publishing to a channel.