A plain-English walk through the number, the green certificate and the residence card.
At heart, the NIE is simply the number. EU citizens are given a green certificate; people from outside the EU are given a TIE card. All three end up being called, confusingly, "the NIE".
If you are from outside the EU and settling on Menorca for the long term, you will in time need a TIE card that carries your NIE number.