A serie de facturación is a self-contained numbering block within which invoices are numbered consecutively and without gaps; you can use a single series or several, for example per business line or per year. Corrective invoices must be issued in a separate series, and the typical mistakes are skipping numbers, repeating them, or changing the numbering scheme mid-year.
Under Verifactu the series matters even more: the invoice number and series identify each invoicing record and travel in the hash chain, so gaps and duplicates become visible to the tax agency. Define your series at the start of the fiscal year and keep them stable all year long.
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