Here's why you need a NISS (Número de Identificação da Segurança Social) in Portugal:
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1. Employment: If you are working in Portugal, your employer requires your NISS to make the appropriate contributions to the social security system on your behalf. This ensures that you are covered for benefits like unemployment, sick leave, and retirement.

2. Self-Employment: If you’re self-employed or run a business in Portugal, you’ll need a NISS to make your own social security contributions.

3. Healthcare: Access to public healthcare in Portugal often requires you to have a NISS. While the health system may also use other identifiers, having a NISS ensures you can access services you’re entitled to, based on your social security contributions.

4. Benefits: If you’re eligible for any social benefits, like family allowances, unemployment benefits, or retirement pensions, the government uses your NISS to manage and process these benefits.

5. Tax Purposes: Although the NISS is different from the fiscal number (Número de Identificação Fiscal, NIF), having a NISS can sometimes be relevant in tax situations, especially when claiming certain tax benefits related to social security.

6. Residency: For expatriates or immigrants living in Portugal, obtaining a NISS can be a part of the residency application process or proof of integration into the Portuguese social system.

In essence, the NISS is a key identifier that integrates individuals into the Portuguese social security system, ensuring they receive the benefits and services they’re entitled to and fulfill their social security obligations.