Endless Modifier System (EMS) the GM rolls D% to determine the difficulty of the task, then multiplies it by .5 up to 5x determined by the impossibility of the task with .5 being easy and 5x being nearly impossible.
Characters on the other hand roll 1d20 and multiiply by their endless skill modifiers. The characters skills can be either as broad or as defined as the player can think of, but only applicable skills can be used to increase the multiplier, but any applicable skills get used.
For example, say the character is shooting a pistol and the GM rolled a 70 (x2) on the opponents duck and cover roll. The character would need to beat a 140 in order to hit their opponent.
Say the character has 2 points in pistols, 1 point in short distance shooting, 1 point in custom weapons, 1 point in combat finesse, 2 points in firearms, and 1 point in behavior prediction that the GM rules all apply. The character would make their d20 roll and multiply it by 8. If the roll is greater than the 140 needed to hit their opponent, they succeed.