It is up to you to decide how to use the departments. You can use only one department Dept 01 and put all your sales there. You can use one department for each customer (assuming you have only up to 8 customers). You can use one department for VATable goods and another for non-VATable goods.
You can use the departments for the various types of goods (such as hot drinks, cold drinks, etc) and keep Dept 8 for the VAT or tax amount.
If you need nine or more departments, then this Pocket Cash Register is not for you.