You can have multiple invoice categories. These categories will allow you to view different groups of invoices, e.g you can view all invoices for services separately if you need to. In order to add a new category, simply scroll down to the invoicing section and open 'Invoice categories'.
You will see a form on the left-hand side and a list of existing invoice categories on the right-hand side. You can add your new category on the left and it will appear on the right when you click Add invoice category.
Click 'Edit' next to the invoice categories if you wish to change them. You will see a page where you can edit the information with a space to name the category. Change this information as required and click 'Save invoice category'.