I did not see any good info in GM Law but here is an educated guess.
From GM Law page 106 "[size=78%]For poisons, the effect is the most severe effect that can be gained with the herb. You must use the tables provided in Section 11.0 (Poison and [/size][size=78%]Diseases) to find the actual effect (based upon the RR of the victim)."[/size]
[size=78%] [/size]On GM Law page 93, it has various charts and info about specific poison's "general" onset and effect. But there are only 4 categories listed of severity, mild, moderate, severe and extreme. But the poisons in the table go from A to E affect wise. If the info in the table went from A to D then I would say that the letters were the max severity a poison started at.
But I could also see the authors cutting the range from 1-5 (A-E) to 1-4 (A-D) for some reason and just missing the clean up in the poison entries.
MDC