Criticals represent the improved outcomes of actions that resulted in an exceptional success. When the result on the die matches the value of the Attribute (remember: the value of the Attribute with all MODs applied), the action is a Critical success. The goal of the action was achieved in the best possible way. A perfect success, congratulations!
Attributes below 1
Negative MODs can sometimes cause the final value of an Attribute to effectively drop below 1 for an action. In these cases, the player does not roll the dice, and the Roll is an automatic failure.
Attributes Above 20
In other cases, accumulating positive MODs can cause the final value of an Attribute to rise above 20. In this case, consider the final value of the Attribute to be 20, but with the peculiarity that the player adds to his result on the die the amount by which the Attribute exceeded 20 (an Attribute of 23 would add 3 to the die).
In these cases, any result of 20 or more on the roll is a Critical, if applicable. Attributes above 20 (either from positive MODs or plainly stated in the troop profile) increase the chance of obtaining a Critical result.