Categories are used for organizing products into logical structures, based on VAT rates.
Cashy-POS allows you defining as many categories as you wish, but only 10 of them can be displayed  on main screen.
Defining a category («Manage Cashy > Category»)  allows you to tag it as «visible on main screen».
Tap and hold one of the visible category buttons on main screen to get a list of products associated to that category. You can filter the list by product number and product name.