I cannot agree with Dave's statement above. 12.1.0.1 clearly states that chapter 12 applies to ALL disciplines. 12.1.0.2 then notes that discipline specific rules over ride chapter 12 unless otherwise stated.
12.8.8.6 then clearly states that a cover is required to prevent entrapment between the lower chain run and the rear sprocket.
Chapter 18 then makes no specification in this regard other than to note (unnecessarily) that a countershaft sprocket cover is required.
So, a rear cover IS required as it is contained in the umbrella chapter 12 and CMX specific Chapter 18 does not make any additional specification.
There can be no other interpretation in my view. If there is, then it means one cannot make a reasonable interpretation of any part of the MoMS, because chapter 12 is VERY clear.
You will note that chapter 17 covering modern MX also makes no specification regarding chain covers, so requiring covers in that discipline must be consequent to chapter 12.
If I am incorrect, please explain how as I can't see any other interpretation and it would be good to have this clarified early.