When an order or invoice row has a product that doesn't exist yet, the product is created. Choose the product category it gets.
Good to know
This is required.
Choose from the product categories in your system.
If the list is empty, check that the connection is set up. An empty list almost always means the connection, not the setting.
If a category is missing, create it first, then click Clear Cache on the settings page so the list is refreshed.