A Ukrainian company has two numbers you will meet on invoices and in contracts. They get confused constantly, though the difference is simple.
ЄДРПОУ
The company identification number. Eight digits, no letters, assigned on entry into the Unified State 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.
VAT number
The tax identification number, in the form CZ 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 ЄДРПОУ, 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 VAT number without being registered for VAT. Registration for income tax and registration for VAT are different things. For a VAT payer, the same VAT number doubles as their EU VAT number.
So
Everyone has an ЄДРПОУ; only those registered with the tax office have a VAT number; and VAT payers are a narrower group still. The fact that someone quotes a VAT number therefore does not mean they are VAT-registered — that has to be checked separately.
How to check them
Check an ЄДРПОУ in the Unified State 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.