Child Forums?

I'm curious, what exactly is "child forums". It comes up as one of the search parameters when you click on "New Posts".

I've tried searching for it but nothing seems to come up.

Quote:

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Search:
Type: Posts; Posts From Last Day; Forums: English Forums and child forums

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parent forum = main forums.
child forums = sub-forums.

Hm, first time hearing parent and child terminology used for forums, but it makes sense. Thanks for clear up.