Most people only need two tax categories.
All inventory items, products and services, are assigned a tax category. By default, POSitive creates Merchandise and No Tax tax categories which should be sufficient for most businesses.
If a type of inventory, such as labor, has a unique tax rate, then you may add additional tax categories. Keep in mind that the simpler you can make it the better.
WARNING: Adding a new Tax Category requires editing of ALL existing Tax Groups so that each Tax Group will have an assigned tax rate linked to the newly added tax category.
(see Tax Rates)