SII or Verifactu: Technical differences and implications for software developers in 2026

In recent months, there has been a lot of talk again about the SII (Immediate Supply of Information). Although this system has been in force since 2017, it is back on the table as it presents itself as an alternative to Verifactu. This raises a question for software development companies: should they now also adapt their systems to the SII so that taxpayers can freely choose which system to opt for?

Throughout this article, we will develop what the SII and Verifactu consist of, their differences, and, above all, how they differ at a technical and operational level, especially from the perspective of integration and data transmission.

What is the Spanish Tax Agency’s SII?

The SII (Immediate Supply of Information) is a system from the Spanish Tax Agency (AEAT) that consists of the electronic submission of billing records that form part of the VAT Record Books via the Electronic Office.

Instead of keeping the Record Books internally and declaring them later, the taxpayer sends this information to the AEAT almost immediately. The goal is to digitize VAT control, reduce errors, and facilitate checks, as the AEAT receives the information with less delay.

Who is obliged to comply with the SII?

The SII is mandatory for certain taxpayers, especially those who, due to their fiscal profile, are subject to higher VAT control and reporting requirements. Generally, it is associated with:

• Entities registered in the REDEME (Monthly VAT Refund Registry)
• Large Companies (turnover exceeding €6,010,121.04)
• VAT Groups

Who can voluntarily join the SII?

In addition to the mandatory cases, the SII also allows voluntary enrollment for companies that, although not obliged, decide to operate under this system for internal management or tax strategy reasons.

What is Verifactu?

Verifactu is part of the framework of the Billing Computer Systems Regulation (RRSIF), developed from the Anti-fraud Law 11/2021. Its objective is to ensure that billing computer systems comply with technical anti-fraud requirements that guarantee, among other points:

• Integrity and inalterability of billing records
• Traceability of data generated by the system
• Chaining and digital fingerprinting of records
• The ability to submit records to the AEAT in "Verifactu" mode

Who is affected by Verifactu?

It mainly affects:

• Companies and freelancers with a turnover below €6 million who issue invoices using a computer system.
• Developers and invoicing software providers, because they are the ones who must offer systems adapted to the regulation.

When will Verifactu come into force?

Following the extension of deadlines, Verifactu will be mandatory for taxpayers in 2027:

• January 1, 2027: Corporate Income Tax (Impuesto sobre Sociedades) taxpayers.
• July 1, 2027: All other taxpayers.

However, the requirement for the software to be ready has been applicable since July 2025 and will continue to apply to any new or updated invoicing software. You can read more details here.

Key differences between SII and Verifactu, in technical and legal terms

Although both involve the immediate submission of fiscal information to the AEAT, they are not equivalent nor do they pursue exactly the same goals. For a developer, the key difference lies not just in what is sent, but in how the data is constructed, with what guarantees, and under which system rules.

Object and purpose of each system

• SII: Its focus is VAT management, reporting the records that make up the Record Books. It is oriented toward tax control and management based on billing records. 
• Verifactu: Its focus is the invoicing software itself, ensuring the system meets anti-fraud requirements. It is oriented toward preventing alterations and guaranteeing traceability and integrity from the source.

What data is transmitted in each case?

• SII: Records are submitted to the VAT Record Books, which include structured information on issued and received invoices, among other categories. 
• Verifactu: Billing records generated under specific rules of integrity, chaining, and system fingerprinting are submitted (issued invoices only).

The practical difference for invoicing software is that in the case of the SII, the focus is accounting as well as fiscal. In contrast, with Verifactu, the focus is structural to guarantee traceability and integrity; that is, it is not enough to send the correct fields, the records must follow a specific structure to prevent tax fraud.

Sending logic and timing

The SII works with a short-term submission logic, directly linked to VAT management and when records are accounted for. This is known as fiscal reporting. Submission deadlines for the SII are as follows:

• Issued invoices: up to 4 natural days from issuance.
• Received invoices: up to 4 natural days from accounting.
• Investment goods: until the submission deadline for the last period of the year.
• Saturdays, Sundays, and national holidays do not count.

In contrast, Verifactu aims for real-time submission integrated with the system's own invoice issuance and registration flow. This makes it an invoicing requirement with anti-fraud guarantees, where the XML is generated at the exact same moment the invoice is issued.

Technical implications for invoicing software

For the SII, the software must:

• Generate the correct data.
• Maintain accounting and fiscal consistency.
• Allow submission within the deadline.
• Manage statuses (accepted/errors) with the AEAT.
• It does not require chaining or signatures.
• Submissions can group blocks of 10,000 invoices.

For Verifactu, the software must also:

• Build records with chaining.
• Maintain evidence of integrity.
• Voluntarily, it can sign the records.
• Prevent the system from allowing "deletion or alteration" without traceability.
• Maintain consistency between issuance, registration, and submission.
• The system can group batches of up to 1,000 invoices.
• Have a declaration of responsibility. This implies that Verifactu impacts the system design itself, not just the submission process.

Real complexity for software providers

If you market an ERP, POS, or any software supporting invoicing processes, the challenge multiplies because your portfolio may contain:

• Customers obliged or adhered to SII who seek to automate the submission of certain Record Books.
• Customers who must obligatorily comply with Verifactu before 2027.
• And in parallel, you will soon need to adapt your software to the future B2B Electronic Invoice (Crea y Crece Law), in addition to regional regulations like TicketBAI or NaTicket.

For this reason, maintaining separate integrations for each regulation usually translates into greater technical complexity, higher maintenance costs, and more friction when updating systems to the new requirements of different regulations.

fiskaly SIGN ES: a single API to cover Verifactu, the SII, and other fiscal obligations in Spain

At fiskaly, we work with a clear idea: fiscal compliance should not force you to rebuild your software every time a rule changes. With fiskaly SIGN ES, software providers can integrate a single API and scalably cover key regulations in Spain, including:

• Verifactu
• TicketBAI
• NaTicket
• B2B Electronic Invoicing (Crea y Crece Law)
• SII

Thus, instead of choosing one integration per regulation, you can build a solid, flexible technical foundation prepared for different taxpayer profiles, with a specialized partner who accompanies you as the regulatory landscape evolves.

With fiskaly SIGN ES, software providers can cover Verifactu and other regulations by integrating a single API, with expert support for developers, clear documentation, and a technical base prepared to adapt to regulatory changes in Spain and Europe.