Product invoicing settings, product groups, and units are managed under Settings > Feature Settings > Products.
Product invoicing settings
You can define whether the product unit and code are displayed on the invoice row.
Product categories
You can add product categories with subcategories.
The main product group is created by clicking Add product category
Below that, you can create a sub-category by clicking the three dots in front of the main product group (...) > Add sub-category
You can create sub-categories on multiple levels.
Product units
By clicking Add unit, you can define different units for products such as hours, pieces, etc.