[FMassSubsystemRequirements](API\Plugins\MassEntity\FMassSubsystemRequirements) & AddSubsystemRequirement ( const [TSubclassOf](API\Runtime\CoreUObject\Templates\TSubclassOf)< [USubsystem](API\Runtime\Engine\Subsystems\USubsystem) > SubsystemClass, const [EMassFragmentAccess](API\Plugins\MassEntity\EMassFragmentAccess) AccessMode, const bool bGameThreadOnly )