6x is for leveling a mythic character from 1 to 99 to reach/buy eternal ascension.
Ascensions - rank - base exp required
1st - Common to Magic - 1x
2nd - Magic to Rare - 2x
3rd - Rare to Epic - 3x
4th - Epic to Legend - 4x
5th - Legend to Mythic - 5x
6th - Mythic to Eternal - 6x
xx - Eternal level 1 to 99 - 14x
Those numbers from the preview are correct except maybe for the last one. Sorry for not being clear 