At the beginning, every player has 50 substitute points (VP). If a player does not have time for an assigned shift, he can cancel it. This costs a few VP depending on the length of the tour. Other players can then take over the shift, earning the VP the driver has paid for the cancellation.
Even if you have no VP, you can still cancel shifts, but this costs money (virtual).
In the long term, this means that you can cancel shifts without problems and penalties, if you also compensate the canceled shifts of other players. Too many cancellations of many shifts lead to fines. This is to avoid that too many tours are blocked by players who do not drive them.
In addition, there is of course a ranking, to compare with the other players.