Use Conversation when users need a discussion thread around a record or workflow item. Use messages and notifications when the user only needs to be told something happened.

Conversation screen type in the builder.
Use Conversation when:
A reviewer needs clarification.
A requester needs to answer questions.
A technician needs to discuss work with a dispatcher.
A finance reviewer needs more context.
A customer-facing team needs a history of messages.
Add a Conversation screen or choose it from a wizard.
Connect it to the record or workflow item.
Decide who can participate.
Decide who receives notifications for new replies.
Make sure messages link back to the record.
Preview as each participant.
Send a test message and confirm the other role sees it.
A conversation setup is ready when each participant can open the thread, read the record context, reply, receive the reply notification, and return to the work item.