There is the Old World card "Greenskins Invade". It states
While this Old World card remains in play, each player battling in a region containing an Event token may roll only one battle die, in total, per battle phase.
It's clear that no matter how many dice I could use in the battle phase, I can only use one of them in this region. But what if I roll a critical (6)? Can I reroll the die because I'm using the same die (so the count of the total dice used doesn't change), or does it count as using a different die?