After a long process I finally have my NIE, only to discover at the bottom of the certificate that it is only valid for three months.  Exact wording: Este certificado tiene una validez de tres meses

Does that mean I have to reapply for a NIE within three months, or is it simply a certificate of proof?
If the latter, I suppose the NIE is now officially mine, but if I need to supply proof I’d need to go back and apply for the cert again - is that right?