Why is PNX divided into light-on and dark-on?

Checking the input / output circuits of PNX-705R [Light On] and PNX-705L [Dark On], both are C contacts, so if you use both gray and white lines, they will be light on, and if you use black and white lines, they will be dark on. I feel like I don't need the concept of light-on and dark-on, but how about it?

The reason why it is lined-up of light-on and dark-on is whether the customer want to check if there is an object there or no object when the power of PNX is turned off. Since it has a contact C, either one can be used, but the operation when the power is turned off must be either light-on or dark-on. * LIGHT-ON = Light interception Light receiving : Relay ON Light interception : Relay OFF Power OFF Relay OFF * DARK-ON = Light interception Light receiving : Relay OFF Light interception : Relay ON Power OFF : Relay OFF

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