Electromechanical and semiconductor relays
Did you know? Coil types used in electromechanical relays
Basically there are two types of relay coil. The most used standard coil is non-polarized - a simple copper wire winding. A strong magnetic field that drives the relay, requires either a high current or many turns - this needs space.
The second type of relay coil 'polarized', combines a permanent magnet with a smaller coil. The magnet supports the EM field of the coil. This reduces the number of turns required so devices can be smaller and consume less energy. Polarized coil systems are the basis of latching relays.