bbPress.org

https://codex.bbpress.org/getting-started-with-bbpress/

Adjust your Forum attributes. These are:
Types: choose whether you are creating a Forum or category.
Forum – can contain topics and be the parent of other forums.
Category – can only contain forums (no topics). Use for organizing your forums.

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You may, for example, have a Category “Open Source Software” with the Forums “bbPress” and “WordPress”.

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Parent: Forums can have a hierarchical structure. If your forum is a sub-forum of another Forum or is the child of a category, add the Parent here.