Reserve capacities on classes allow you to hold a certain amount of seats for a certain subset of students. Sometimes classes will have multiple reserve capacities listed per class. The trick with adding multiple reserve caps is that you cannot have two reserve capacities on the same row. The system allows you to do this, but you should not.
You have to use the top plus/minus button on the tab, then add the new reserve cap information on the next row (the reserve capacity sequence will advance by one).
Here are examples: