A Luxembourg company has two numbers you will meet on invoices and in contracts. They get confused constantly, though the difference is simple.
numéro RCS
The company identification number. Eight digits, no letters, assigned on entry into the Trade and Companies Register or the trade register. Every registered entity has one — companies, associations, foundations and sole traders alike.
It never changes — not on a change of name, address or ownership — and it is not reissued after a company is struck off. That is what makes it a reliable way to identify a company years later.
numéro de TVA
The tax identification number, in the form LU plus eight to ten digits. It is issued by the tax office on tax registration, not by the register. For legal persons the numeric part is often the same as the numéro RCS, but that is not a rule and cannot be relied on — for individuals it almost never holds.
An important detail: an entity can have a numéro de TVA without being registered for VAT. Registration for income tax and registration for VAT are different things. For a VAT payer, the same numéro de TVA doubles as their EU VAT number.
So
Everyone has an numéro RCS; only those registered with the tax office have a numéro de TVA; and VAT payers are a narrower group still. The fact that someone quotes a numéro de TVA therefore does not mean they are VAT-registered — that has to be checked separately.
How to check them
Check an numéro RCS in the Trade and Companies Register or in VMI: enter it and see whether the company that comes back is the one you expected. Check a numéro de TVA in VIES, which answers for the whole EU and tells you whether the number is valid right now. Both are free and take seconds.