Is NISS mandatory for my AIMA appointment?

Short answer: For most residence-permit routes, yes — and you can apply for NISS remotely via Segurança Social's portal once you have a NIF.

The NISS (Número de Identificação da Segurança Social) is required if your visa carries a work or self-employment dimension (D2, D3, D7-self-employed, D8). Pure passive-income D7 sometimes proceeds without it but AIMA increasingly asks. Start the NISS process as soon as your NIF is issued — see Your NIF.