On all maps where you have built a depot, you can directly drive all lines. Additionally you can also make driving exams on these lines to earn more money on the line. This costs a unique fee, depending on the size of the line.
The exam can be driven with any bus from your depot.
At the end of the trip, you get a normal evaluation. The better the exam, the higher is your bonus for this line. If you complete an exam with 100 %, you will from now on get the doubled salary on this line. An exam can be done as often as you like, to try to improve your bonus.