Hola. I’ll be living and working in Portugal from this summer as a self-employed/freelancer. When and for what reason do I “use” my número de contribuinte in situations when I’m asked upon paying at restaurants, supermarkets, etc.? Is this meant to be used as a kind of receipt for business expenses or something? Could someone please explain, as I don’t clearly understand the purpose of it.
@siriusumare, the government offers a certain percentage of tax deductions for everyone who asks for receipts with their número de contribuinte. Without this number, the expenses and respective deductions can’t be linked to you when you file your tax return. Any and all expenses are valid (business and personal), although they’re grouped into different expense categories that each have their specific deduction percentages and caps.
The main purpose is to combat tax evasion from businesses who don’t properly declare their profits, and some of the aforementioned expense categories specifically target businesses that tend to be non-compliant, such as repair shops, restaurants and hair salons.
In any case, as a freelancer, depending on how your accounting is managed, you can also use this to deduct business expenses, if applicable 
You should look into this in detail with an accountant/someone who can give you qualified advice for your specific situation.
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