I assume this means supplemental rules for critical hits. There were all kinds of things from Judges Guild booklets and tables in Dragon Magazine, etc. We developed our own systems for critical hits. Some of our systems were complex and specific as to what body part got injured to instant death. This worked for both critical hits and critical fumbles 1 on d20. No matter what a 1 always missed.
If we rolled a 20 or a 1 on a 20 sided die, we would roll again and second 20 or a second 1 would result in a critical hit for a 20 or a fumble for a 1. There were many awesome hits and fumbles. People shot themselves with their own bow and arrow, or bow strings broke, or people cut off body parts, etc.
The simplest of these was just rolling double damage to keep things moving.
A few of those who DMd would go all out with these materials. Others decided it slowed things down too much for the intricately detailed tables.
I do remember one time that someone wrote to the small town paper complaining about a Judges Guild booklet that had a picture of a woman on an altar and said that proved D&d was evil. The guy from our club who now owns his own game shop, Phil, wrote back and I will never forget this line. “Calling Judges Guild D&D is like calling football Spaulding.” Of course that was back when the D&D was evil was very powerful.