appEui unclear 

I'm not sure about this AppEui. Should we provide a function where the AppEUI can be read from the device:  
`  String appEUI() {  
    return getValue("APPEUI");  
  }`

or is this not necessary? 
I don't understand the appEUI is this a number I can invent? Perhaps there is some Documentation needed. 
Is using the one Programmed into device not sufficient? 