Difference Between High And Low Impedance Relay.

Dear All,I need some clarification on the difference between High and Low impedance relays. I learned that a High impedance relay has a large series resistance connected to reduce any through fault current during transient condition to avoid unnecessary relay operation.Is it true? IF yes then why don't we add a series resistor to a Low impedance relay to avoid false operation? Also I came to know that HI relays will have all the connected CT's of the same ratio. Why is it so??what are the specific locations where we use HI and LI relays? Kindly clarify. looking forward for the earliest reply.