Output orders can be used in non-WHS (R3 Warehouse management) scenarios. They are created when you post a Sales order Picking list (or they can be created manually, but I have never seen anyone actually do this). If the order is not reserved when you post the Picking list, it seems you can't make a reservation against the order. But you can go to Picking list registration and make the reservation there.
And if you're using 'Consolidated picking', then output orders feed into something called Shipments (in the Inventory module), and you can do things with reservations from there too.