I think you're referring to a facility Nabble calls sub-forums.
Try exploring. Create a forum as a test and play. You find where you can create sub-forums at:
Options > Structure > Create new sub-forum
Also take a look at:
Options > Application > Change application type
That provides this set of options:
Forum
Page with topics and discussions.
Mixed
Page with topics grouped by sub-applications. If no sub-apps exist, a normal list of topics is displayed.
Category
Page with a simple list of sub-applications and discussions.
Board
Page that groups sub-applications and discussions.
Gallery
Photo and image gallery.
Blog
Page with full messages and related comments.
News
Page with headlines and posts.
Unfortunately those descriptions are pretty meaningless. That why I say create a test forum of your own and explore.
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Once the admin for GregHelp now deleted.
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