Residencia does not exist any more, it hasn't existed for a number of years although the term is still used.

'Certificado de registro de ciudadano de la union'  translates to 'Certificate of registration of citizen of the (European) union' and not 'Resident of Spain'. It has your Spanish address and your NIE number. It does not oblige you to live at that address or be registered as living at that address - this is the 'Empadronamiento' (or Padron), equivalent to the UK 'Electoral register' and you should only be registered this way in one place and should re register at the new town hall when moving house.

If you reside in Spain for more than 3 months (I guess this is why the NIE has a 3 month expiry) then you should apply and obtain a 'Certificado de registro de ciudadano de la union' certificate.

Official information here:
http://extranjeros.empleo.gob.es/es/InformacionInteres/InformacionProcedimientos/CiudadanosComunitarios/hoja102/#aplicacion

I've searched to see if you should cancel the certificate if you are not living here but there is nothing on any of the official government sites.

Unofficial and correct/incorrect information and interpret how you want: Search Google

Personally I have a 'Certificado de registro de ciudadano de la union' as I live here permanently but if I didn't I would still get one to save the hassle.