This is an identification number for use by foreigners in Spain. You are required to have a number if you have dealings within Spain, even if you are not full time or tax resident.

You can get a number from your local Police Station in Spain at the foreigners' department. Depending upon the Local Parliament in which the property is located, NIE will be compulsory to pay the taxes. If the voluntary period to pay the taxes is exceeded a fee will be charged. Your local lawyer can advise.