A no-jargon walk-through of the number, the green certificate and the residence card.
The NIE is the number, full stop. EU citizens receive a green certificate; non-EU citizens receive a TIE card. All three keep getting lumped together and called "the NIE", which is where the confusion starts.
If you are non-EU and putting down roots in Fuerteventura for the long term, the document you will ultimately hold is a TIE card showing your NIE number.