A jargon-free breakdown of the number, the green certificate and the residence card.
The NIE is the number. EU citizens collect a green certificate; non-EU citizens collect a TIE card. All three end up lumped together — confusingly — as "the NIE".
If you are non-EU and settling into Sevilla for the long term, you will eventually hold a TIE card with your NIE number printed on it.