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

Conclusion:

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).