This falls under the topic of coordination or selective coordination. When you have multiple OCP devices in a circuit, say a Switchgear feeder breaker feeding a Load Center Panel feeding a Branch Panel, you use the curves to insure that the breaker closest to the load trips openrather than trip open a breaker upstream. Generally, you can get three series breakers to cooperate and no more because of curve overlaps. When the curves are uncertain or there is too much "slop", or you are looking to trip open on something other than over-current, then you use relays driven by special sensing devices.
