The main oil difference between JASO and other standards is that JASO adds variant kinds of anti-slip friction coefficient agents in engine oils. So,the engine oils are suitable for all types of motorcycles and the purpose is to stop wet clutch from slipping.

JASO MB= Engine oil specialized for Scooter with Dry-Clutch
JASO MA=Engine oil specializedfor Motorcycles with Wet-Clutch
MB is more efficient at saving fuel than MA
The viscosity of MA2 is higher than MA1

MA is suitable for motorcycles with wet clutch.
MB is suitable for scooters with dry clutch.
If you need to choose between MA1and MA2:
MA1 is applicable to <= 500cc motorcycles.
MA2 is applicable to > 500cc motorcycles.
MB can be used for Scooter (dry clutch) only.
MA can be used for both Motorcycle (wet clutch) and Scooter (dry clutch).