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On the right side of the table, you can sort the categories (either using the arrows, or by dragging the row around), and disable the ones you want hidden from the customer.
All the categories are actually sub-categories of the "Home" category.
To create a new category (or a sub-category of an existing category), click on the "Add New" button from the any level of categories.
The form is the same when creating a root category or a sub-category. The only difference is that when clicking "Add New" from a sub-level of categories, PrestaShop understands that you want to create a sub-category, and therefore sets the "Parent category" option accordingly.
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