A Slovenian business has three identifiers you will meet on invoices and in contracts. They get confused constantly, though the difference is simple.
Matična številka
Ten characters, assigned on entry into the Poslovni register Slovenije. Every registered entity has one — companies, sole traders, associations, institutes and foundations alike.
It never changes — not on a change of name, address or ownership — and it is not reissued after an entity is struck off. That is what makes it a reliable way to identify a business years later.
The last three characters identify a part of the entity: 5043611000 is the company, 5043611001 one of its branches. For entities with very many branches the counter spills into a letter — Mercator's units run 5300231A01, 5300231C34. So never treat a matična številka as a number.
Davčna številka — the tax number
Eight digits, assigned by the Financial Administration on entry into the tax register. Every taxable person has one, including those not identified for VAT, and including individuals.
The VAT identification number
The tax number with an SI prefix — SI plus eight digits. Only an entity identified for VAT purposes has one. Registration for income tax and identification for VAT are two different things.
An important detail: an entity can have a tax number without being liable for VAT. So the fact that someone quotes a tax number does not mean they charge VAT — that has to be checked separately.
So
Every registered entity has a matična številka; every taxable person has a tax number; and those identified for VAT are a narrower group still.
How to check them
Check a matična številka in the business register: enter it and see whether the company that comes back is the one you expected. Check a VAT number in VIES, which answers for the whole EU and tells you whether the number is valid right now. Both are free and take seconds.