MLSS=MLVSS+ non volatile mixed liquor suspended solid.
MLVSS actually indicates the concentration of microbes present in the system to degrade the waste.
Measuring MLVSS you can obtain the F/M ratio, SVI value (Sludge Vol. Index)
Ant wastewater treatment plant should attain a critical F/M and SVI value to work properly. For better understanding you should go through Wastewater Engineering by Metcalf and Eddy or other books...