Within the back office, it is possible to add a tracking number to an order.. Firstly, If you go to your delivery methods under 'Shipping'. Once you have selected the delivery method you will be shown the page below where you can choose which how you want the tracking code to be generated. Once you have changed your setting simply click on 'Update'. 

The first setting will allow you to manually add a tracking code to the order, which could come from your courier/Royal Mail. Once you have received the tracking code you can the 'Item Dispatched' option in the drop down menu. This will display a content box where you type the tracking code in. You can tick the box to notify the customer that there order has been dispatched and that they can track their order with the tracking code as shown below. 

The second method is a system generated tracking code. When a new order has been processed and dispatched, the system will generate a tracking code which is the order item number. This tracking code can be included in the email to the customer when dispatched as long as the custom tags have been added into order process emails.