ROI is the Register of Intra-Community Operators, which must be requested via Modelo 036 in order to invoice EU businesses without VAT, and VIES is the EU wide system used to check whether a company's VAT number is registered. Before invoicing an EU business customer without VAT, it is worth checking their number in VIES.
If the customer does not appear in VIES, the safest option is to invoice with Spanish VAT until they sort out their registration.
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