Once you have selected to add a new item or selected an already created item, you will be within the 'Extended Item Setup' page.
Here you will be able to assign a routing group to a product. These routing groups will be the ones created in the 'Kitchen Routing' menu.
As you can see this is not a mandatory field but will be useful in organising printing i.e. a soft drink within the 'Drinks' routing group will only print to the bar.