Control relays are magnetic devices that are used to remotely switch loads. They are functionally identical to a magnetic motor starter, save that control relays lack horsepower-rated contacts and built-in overload protection.
Since magnetic-motor starters are limited in the number of auxiliary electrical contacts, control relays are used where additional contacts are required to switch low-level current loads such as pilot lights, motor starters, or other relays.
Some control relays are the latching-type relay, in that after energizing the coil, the relay contacts remain mechanically latched in the closed position, even after power is removed from the coil. To open the relay contacts, the coil requires the momentary energization of a second coil to unlatch the coil.
The advantage of this type of relay is that it is not necessary to keep the coil continuously energized, which results in quieter operation and less current drawn from the source.
Reference
Basic Motor Control by Aaron Lee and Chad Flinn is used under a CC BY 4.0 Licence.