To get the most granular data for the most actionable insights, it is best to train Agents to ensure they always apply Topic(s) to their Conversations. You can also set up a Rule to automatically apply the most detailed sub-topic when applicable.
By default, Conversations can be closed without a Topic, but you can make it a requirement through the Conversation Workflow settings page. In the below screenshot, see how this will appear to Agents when the setting to require Topics is required:
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