Discussions are kind of like old school web forums. Instead of a stream of instant messages, discussions organise messages into topics, or threads. Rhub will keep track of when a thread has been updated, so you don't have to keep checking up on it.
For more urgent topics, rhub lets you discussions out to people's phone, email or instant messager. You can make these discussions opt-in or opt-out. For example, you might use a opt-in discussion to organise an event, so that only people interested in it get messages.
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