Major Effect Dice are designed to be used with Cypher Systems games such as Numenera and The Strange.
From the campaign:
Major Effect Die are for your Numenera and The Strange games, or any Cypher System game. They show the challenge level you reach instead of a number 1 to 20 then divide by 3. So they make your game goes faster, no math!
Have a +1 or +2 to your roll? Add it to the number of Cyphers on the die, if it’s over three add one to your challenge level!
We even give you the + to damage right on the die.