You can use it to hook up an additional open & close sensor for each input. This is an optional feature and it is not required to use Nold. This sensor can be a cheap magnetic contact switch, for example: 

This switch should be NC (Normally Closed, which means if your garage is closed, this should be closed too). You can buy it at your local hardware store or order online (e.g. from Amazon). The reason why it could be useful with Nold: If you are using the auto-open function and your door is already open, it won't close it when you are coming home. Later this input will be used to check the status of your door remotely and even control it remotely. 

Sample placement for garage doors:

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