zulip/templates/zerver/help/require-topics.md

Require topics in stream messages

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By default, users are not required to specify a topic in stream messages; if a user sends a message without a topic, the message's topic is displayed as (no topic). If message editing is enabled, any other user can then edit the topic of such a message to set a topic, even if community topic editing is otherwise off.

You can instead choose to require a topic for new stream messages.

{settings_tab|organization-settings}

2. Under Message feed, select Require topics in messages to streams.

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