Occasionally if a couple purchase a property together one spouse’s NIE number is used as the lead on subsequent documents and the other spouse’s number never becomes activated with the Agencia Tributaria (Tax Office), or if an individual gets an NIE number at the Extranjeria and then for whatever reasons never uses the number for anything (i.e a house or car purchase, importing anything over 150 euros, getting a job, becoming self employed etc. etc.) their number too remains unactivated.
This is easily rectified and you should take the trouble to sort it out in case you need to use the number in the future for any tax related issue.
Once completed you need to present it by appointment at the Agencia tributaria – take the modelo, and your passport and NIE and a copy of each or you can authorise someone to do this on your behalf.