Here is an attempt of real world scenarios which can happen for flight and airplane
1. A flight going from Toronto to Las Vegas can stop at Denver. You have same flight number TOLA123. You do not come out of the flight. The plane is halting for passengers getting in and out only. Depending on the destination travelled you pay different amounts.
2. A flight going from Toronto to Las Vegas can stop at Denver. You have same flight number you leave your luggage inside 'come out get in' all allowed but passengers from Toronto take the boarding pass and use it for reentry , get one ticket and the flights are two in the system. One is TOLA123 and second is DELA123, airplane is same.
3. Two flights going from Tornto to Las Vegas you have to get down and get in into a new airplane - it is simple as two flights, two airplanes, two tickets and offcourse two classes.