Premium opens learning, practice and mock exams, in every exam you study, for as long as it runs.
With Premium you play every unit and lesson, get your reviews and your plan up to the exam, practise, and sit as many mock exams as you like. It belongs to your account, not to one exam, so it works in every exam you prepare, including one you have not bought.
Buying an exam is separate: it gives you the whole book and your scores for that exam, for good. When Premium stops you keep those, and you can still open the mock exams you handed in.
Every exam you buy comes with a period of Premium, shown before you buy. After that you can subscribe, monthly or yearly, on licento.com. If you subscribe while Premium is still running, you pay nothing until it runs out.