1. The mathematical statement says,that, if the input is "bounded" that is within finite values ,then the output of the system, is also bounded. for a stable system.
2.In real systems , it is conceivable that the system is stable but the equipment fails if any component exceeds its specification. For example, in even a simple operational amplifier, if resistors are not chosen for proper heat dissipation , it might fail ,even though the circuit values assure a stable. response from a control point of view.. One other example is power system stability , where we encounter a breakdown if the torque angle exceeds a critical value during disturbances..
3 Another possibility is that the given definition ,could include limit cycles and attractors in chaos situations.
4 A possible application of summing n terms of input and output would be in Power system stability . Consider a sudden increase of load . in a power network. If we are able to sum power input and output at each node and impose some thresholds we can come up with a criteria for ensuring stable operation in real time.