You can create your own categories for products, contacts and tasks in your settings under system, where you’ll easily find the category settings.
Afterwards you can add products, contacts and tasks to the different categories.
For products and tasks you can assign a category name, while you can add colors and shortcuts for contacts.
In the contact overview you’ll see the color and the shortcut next to the name of the contact.
This way you can quickly recognize which category the contact belongs to.
If you add a line item to an invoice via “+ Multiselect products”, a window will open where you can filter by category.