That the STEL Order reference matches the folio assigned by the SII is complicated, you can configure it so that the SII folio and the STEL Order reference are the same, however, the SII does not always assign the folio in order, since a pending folio can be assigned and a mismatch between invoice reference and fiscal folio is created, they are two different numbers.
To configure or change the number sequence of the STEL Order reference you must do it from: Settings > Basic settings > References.