The invoice number is the unique code that identifies each invoice and the difference from any other. For the invoices made to have an official validity, the Law requires that they must have an invoice number, so that there cannot be two invoices with the same number.
The bills They must be ordered by the invoice number and there cannot be digit jumps. In addition, the order must coincide in time with the dates of each of the invoices. In this way, the order of the invoices by number and by dates of completion must be exactly the same.
Companies often use number and serial to identify invoices (D21). With the letter they indicate the type of invoice that it is and the number marks the order of realization.
However, companies are free to start with the number they want, as long as they are correlative. Every year you choose the numbering mode and start over.