The change came with Royal Decree 1065/2007, which unified tax identification under the NIF for both individuals and companies. On forms, bank paperwork, and invoices you will see both names used interchangeably; the number is the same.
In practice your SL's NIF is assigned at incorporation: first on a provisional basis and then as final once the company is registered at the Registro Mercantil. It is the number that must appear on your invoices, on the returns you file with the tax agency, and on contracts with banks and suppliers. An autónomo has no CIF: their tax ID is their own personal NIF, the one on their DNI or, for foreigners, the NIE.
Typical mistake: a client asks for your «CIF» and, as an autónomo, you think you need to apply for a new number. You do not: you give your NIF. Another common slip is getting the letter or the check character wrong on an invoice, which can cause trouble for your client when deducting the VAT.
It is linked to the NIF, the census registration return, and the VAT NIF for intra-EU transactions. In kontora the NIF of each client and supplier is stored on their record, and Verifactu invoicing includes it on every invoice.
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