JASO MA oil is for scooters or motorcycles with wet clutches. The piston, gearbox and clutch are all sharing the same engine oil. There is special friction modifiers to ensure all 3 components work perfectly and protected. MA oil is specially designed to prevent the clutch slippage.
JASO MB oil is for SCOOTERS that do not have wet clutches. MB oil is specially designed for more fuel efficiency and less burn.