IRRenewal seems to be right. It should be something related with drift removal. But I do not get it. The newer versions of 737 are more restrictive than the older ones? They lost the technology they developed for the 300? Or is just as IR says- Boeing philosophy “pay us more money “. Instead Airbus has put it very frankly with MABH in their manuals.
Anyway the question remains what is this MUH?