The Real Madrid had a great start to the season after winning the Supercopa de España and Europe. The whites aspire to win all the titles they have yet to compete for (La Liga, Copa del Rey, Champions League, and Club World Cup) in order to achieve the coveted 'sextete'.
However, Barcelona fans believe this is impossible because Madrid did not achieve the 'treble', and without the 'treble' there is no 'sextete'. Why? The answer is simple, the white team only won La Liga and the Champions League last season, and lost the Copa del Rey.
A few days ago, they won the Supercopa de España and Europe, titles that are counted, by some statisticians, as part of last season (2016-2017). From this perspective, Real Madrid can only win five titles (and not six), in case they also win the Club World Cup 2017.
If they win La Liga, Champions League, and Copa del Rey of the 2017-2018 season, they will be counted as part of the new season. To achieve the 'Sextete', as Barcelona fans see it, they must also win the Supercopa de España 2018, Supercopa de Europa 2018, and Club World Cup 2018.