In some applications, a decision is made to limit the magnitude of current that can flow to an earth fault. For example, that would commonly be done on a generator in order to minimize the amount of damage that might be done inside the generator by an earth fault. This is normally done by inserting an impedance between the generator neutral and earth.
But when the earth fault current is intentionally limited, it is more difficult to detect a fault. Hence, there is a need for a special form of earth fault protection that is designed to work properly when the earth fault current is restricted. Hence - restricted earth fault protection.