NISS

Número de Identificação de Segurança Social — your social security number, needed for employment, self-employment and any social benefit.

Where people slip: it is a separate number from the NIF and does not come with it. You cannot be put on a payroll or register as autónomo without one, and requesting it usually needs your residence process to be under way — so it becomes the quiet blocker between "I have a job offer" and "I am actually employed".

Do this: request it through Segurança Social Direta or at a desk as soon as your residence file allows, and give the number to your employer or accountant in writing. It stays with you for life, so keep it somewhere you will still find in five years.