Premium usage is the usage of third-party services. A few examples: purchasing public transport tickets with SMS, SMS parking, ringtones, 0900 numbers, etc. These rates are established by those third-party suppliers. Mobile Vikings only collects these amounts (including VAT) and transfers them to the supplier of such a specific service. Therefore, this usage will be mentioned separately on your invoice applying 0% VAT.
Please note: as of 1 January 2018, you won't receive anymore invoices for prepaid SIM cards. More info here.