Digital output provides functions for turning electric device ON or OFF. The Applications are ranged from driving a relay to turning ON/OFF lamp solenoids, contactors, motors, or alarms etc. In application for latching outputs, a "1" typically causes the associated switch or relay to latch, while a "0" causes the switch to unlatch. Devices can be turned on or off, depending on whether the external device is need normally open (ON) or normally closed (OFF). 

Same to switches, relays are always drawn in the off are un-operated position. Contacts may be drawn in different ways as shown below; but the middle contact (labeled 2 below) is the wiper arm and will move when the relay is energized. This contact is called the changeover contact, abbreviated CO. The top contact (labeled 1) is the normally closed, NC contact (relay not energized) and contact 3 known as normally open, NO. When power is applied to the relay coil, the NO contact is then mechanically and electrically contacted to the CO contact.