A clear account of the number, the green certificate and the residence card, with the confusion stripped out.
The NIE is the number. EU nationals receive a green certificate; non-EU nationals receive a TIE card. All three are loosely, and misleadingly, called "the NIE".
If you are a non-EU national settling in Bilbao for the long term, the document you will ultimately hold is a TIE card displaying your NIE number.