On July 1, 2017, the Immediate Information Delivery System for Suministro Inmediato de Información (SII) became a mandatory requirement for Spanish companies. The SII system is an electronic system for sending information relating to issued customer invoices and received supplier invoices.

Taxpayers covered by SII are required to keep the following VAT registration books (Libros Registros de IVA) through the electronic delivery of billing records to the AEAT (La Agencia Estatal de Administración Tributaria) electronic office:

Companies in Spain must submit invoicing details to the tax agency; this information is used to update the registration books.

The information must be submitted online, specifically through Web services based on messages exchanged in XML format.

Each declaration has a common header with information about the holder of the registration book, and the year and period in which the transactions are recorded. This header information must be accompanied by a section with the contents of the invoices.

For each declaration, the AEAT Web service sends an XML response with the status of each invoice in the declaration and, where applicable, an error description relating to the overall submission or an individual invoice.