Are NUSS, NSS and NAF the same number?
NUSS and NAF are two names for the number Social Security uses to identify you. The pages checked here do not describe NSS as a third, different number. NSS is the initials of Número de la Seguridad Social, which is the phrase those pages shorten to NUSS.
What it is
The Import@ss request page (opens in a new tab) states it in one sentence: the Número de la Seguridad Social (NUSS) is the number with which Social Security identifies you, and it is also known as the Número de Afiliación (NAF). The practical guide for self-employed work (opens in a new tab) asks for the same pair when you register: "your Número de la Seguridad Social (NUSS) or Número de Afiliación (NAF)". It does not ask for two numbers.
The Social Security magazine (opens in a new tab) asks the question "Is the NUSS the same as the Número de Afiliación (NAF)?" and answers that the NUSS coincides with the NAF. It adds a timing detail Import@ss does not spell out: the Social Security number becomes the Número de Afiliación at the moment the person starts, for the first time, a work activity that brings them into the Social Security system, in any scheme. So the magazine treats them as the same number, and it uses "affiliation number" for that number once work has started. It does not tell you to apply twice.
A headline on the same magazine site says "Guía para la solicitud y consulta del Número Único de Seguridad Social (NUSS)". "Único" is the extra word in that headline. The request page itself uses Número de la Seguridad Social (NUSS). Neither page, nor the practical guide, prints a separate field called NSS.
That is why a form, a payslip or a conversation can seem to use three abbreviations. NUSS is the abbreviation the request page puts in brackets after Número de la Seguridad Social. NAF is the abbreviation it puts in brackets after Número de Afiliación, and it says the NUSS is also known by that name. NSS is not given its own definition on these pages. It is the same three Spanish words, Número de la Seguridad Social, written as initials. This answer does not claim that a form has a box labelled NSS with a different value. The pages do not say that.
The longer definition of what the number is for, and the fact that asking for it is not the autónomo alta, is What is a NUSS?. Modelo TA.1, which the request page names when a parent asks for a number for a child, is What is modelo TA.1?. The worker forms list (opens in a new tab) titles Form TA1 as an application for Social Security affiliation, a Social Security number, or a change of data. It does not list a separate NSS form.
What is different in Euskadi
Nothing in these Social Security pages assigns a different abbreviation, or a different number, in Álava, Bizkaia or Gipuzkoa. The Hacienda Foral does not issue this number. If a Basque tax form asks for a NIF or a NIE, that is a different identifier, explained on What is a NIE?, not a local name for the NUSS.
Where to read the steps
Finding or requesting the number is Get your Social Security number. The short glossary line for the same abbreviations is in the glossary. This page does not repeat the request steps.
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