Short and Sweet, It depends on which carrier your ticket is on. At UA you WILL NOT be left behind if there is a seat on the plane(weight restrictions excepted)! If you are a UA employee you can ride in any cabin, onloaded after all rev upgrades are done. If you are from another carrier you are generally alloted only ECY although you could be put in business class or they would upgrade someone else to get you on the flight. Good luck in your travels! Dont forget if you have an ID 90 ticket you can generally get it endorsed and use it on another carrier if airline A is full.