Expand all categories and subcategories. In the same row of the subcategory is the button "A-Z." When clicked, it will sort products in that subcategory in alphabetic order. If "A-Z" is clicked in the category row, it will sort subcategories, but not the products in the subcategories. The "A-Z" button on product page itself will only sort categories.
After every change, save button must be clicked in order to save changes.
Also, this function not only sorts products on the iPad POS but also the Management Console.