Lead single “Roar” has already sold more than five million singles worldwide. It’s certified 6X Platinum in Australia (where it’s topped the chart for nine consecutive weeks), 4X Platinum in Canada (where it has become the biggest-selling single in chart history and has been the No. 1 airplay record on three formats for five weeks running) and 2X Platinum in New Zealand (where it’s been No. 1 for 11 consecutive weeks). “Roar” has gone Platinum not only in the UK but in Italy, Mexico, Norway, Ireland and Sweden and is certified Gold in 10 countries.
It's not the biggest selling single ever in Canada though. I don't know what they mean by that. I think they mean it had the biggest digital weekly sale ever.