You may have heard the term "coil tolerance" or "coil voltage tolerance" and not known specifically what it referred to. And while the concept is simple, it is very important to understanding how electricity operates and further which contactor coil voltage would be best for your application.
Electrical voltages are not exact; they vary based on numerous different factors. As such, the coils within contactors, relays and trip units have a built in variance which accommodates for this potential differential in voltage draw. As a rule of thumb, one can assume a voltage tolerance of + / - 15% on coil voltages. So, for example, a 120 volt coil will effectively work from 102 volts AC up to 138 volt AC and a 240 volt AC coil will work from 204 volts up to 276 volts AC.
One important factor to consider is that while these coils will work for varying voltages within that 15% range, the further the voltage varies from the target coil voltage, the shorter the lifespan that it will have. As an example, if you have 208 volt control wired to a 240 volt coil, technically it will work, but its lifespan will be reduced quite significantly. Furthermore different manufacturers offer different tolerances, some as low as 10% while others claim an acceptable rate of 20%. Always verify with the manufacturer to determine what coil tolerance is acceptable for the control that is being used.