Does the D8 visa require my income to come from outside Portugal?

The whole concept of the D8 is remote work for a business or clients located abroad while you live in Portugal. Whether you are an employee or self-employed, the earning relationship is with entities outside the country.

This is what separates it from local work visas: you are not taking a Portuguese job. You will typically prove the arrangement with an employment contract or client contracts, payslips or invoices, and bank statements showing the foreign-sourced income arriving regularly.