By default in WHMCS, only one level product categories are possible. Assume that you have a category
US and another Category Spain. And under US you have another category California Items
and Under Spain you have another category named Madrid Items.
Then currently you have to create two categories named California Items and Madrid Items as two categories. Then you can add products under California Items and Madrid Items. But there is no way to add subcategory in WHMCS. We developed a module, which allow to add a parent category and then number of child category to it.Anybody looking for such a module, please contact.