In the Orders module, there are two types of order. Normal, and Manual.
NB: Some Tracker clients opt to turn off 'Manual' orders, so you may not see this option when creating an order.
A 'manual' order is a way to create an order which isn't linked to any Products. This can be useful for samples, one offs, or for service-type orders.
A Manual order has to be managed through the order process, so it is marked complete / dispatched by a user actually setting the status, whereas Normal orders are completed when stock is allocated to them.