In traditional forum packages, like vbulletin, phpbb, ipboard, forum boards can be grouped together.
For instance, Mobile phone as the main board, android, windows phone, blackberry can be it's sub-board.
In NodeBB, subcategories are referred to as child categories. In order to set up your subforum, you would create your necessary categories (one for mobile phones, and then others for android, ios, etc) and then set the parent category of your intended subforums to be your mobile phones category.
