VAT Number / Self-Employment Number
Plain English
It's your VAT number, which you need if you want to invoice clients as a freelancer or run a business in Italy. You apply once at the Agenzia delle Entrate (it's free), and you get an 11-digit number that goes on every invoice you issue.
Why it matters
Without one, you can't legally bill clients on a regular basis, register with the right professional pension fund, or qualify for tax regimes like the regime forfettario. It also changes how you pay taxes and INPS contributions.
Real example
"When Ravi went freelance as a UX designer in Bologna, he opened a partita IVA online through a commercialista. The number was active in 48 hours, and within a week he sent his first electronic invoice through the SDI."
Common confusion
Opening a partita IVA isn't just "a tax thing." You also have to sign up with INPS (or the pension fund for your profession) and start paying social contributions, even in months you don't invoice much.
Official source
Agenzia delle Entrate