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Low impedance relays are used to switch high currents(like motors) while high impedance relays are used to switch low currents
The reasoning is as follows.
Relays used to switch higher currents need thicker contacts (to minimise heat and power loss), so the thicker the contacts are the heavier they become. Now to attract the contact (to achieve switching operation) the magnetic attraction has to be more powerful (because of the additional weight), implying the relay's coil should generate a higher magnetic flux. To have a greater magnetic flux, the current through the coil should obviously be higher. To facilitate higher current in the coil the resistance (impedance) of the coil should be lower.
Low impedance relays are bigger and more expensive, so try to use higher impedance relays wherever possible.