I believe it is the number of times that you have your super. Though the footage is split up the hunter uses his golden gun ~3:00 in the beginning of the room and has his super still without a x2 immediately after it shifts back when the ghost is deployed. Also, when it shifts to the titan in this area it shows his super bar reach fully charged and then get the x2 multiplier. I have seen some discussion on this previously and I think the most popular consensus was you could stack your super charges but this feature was removed. But this does not look like it is the case further on. The Hunter gets x2 after sniping an enemy, so perhaps it is a damage multiplier? Gunslinger has no pertinent skills to affect the super like this. However, the first time the hunter supers, again ~3:00 his super bar does not decrease while he is using it. At ~4:00, after just achieving the x2 the bar decreases. Maybe if you keep progressing after you get x2 if refills your bar again and then you get the 2nd charge? Not sure. The Warlock reaches x3 and x4. Perhaps it is damage or regen multiplier, as his bar goes all the way down. Hunter switches to Bladedancer super mid combat although he has the throwing knife melee characteristic of only gunslinger. He does a fist bump mid air. Not sure what this is indicative of. This is all relative to developmental mindsets back in April when the video released. This is all speculation. If anyone has confirmation about the nature of any of this, would love to hear more.