You can travel in a lower class for a particular route if the circular ticket booked class is not available on that route. For eg. If you have taken CJT for 3ac you can travel from Mumbai to jodhpur in 3ac but if you want to go to jaisalmer from jodhpur and 3ac is not available on that route then you can travel in SL. CJT is not given in rajdhani, shatabdi, duronto, garibrath.