There's only one internationally recognized grading system and that's the member organization known as the UMF origination.
The Unique Manuka Factor is a 4-part chemical analysis conducted by a third party (not an inhouse test and not just a test of Methylglyoxal (MGO) which can easily be manipulated). The 4 in 1 UMF test looks for:
Methylglyoxal (MGO) levels which is the unique chemical found only in Manuka which makes it so immune-potentiating for humans to eat and this signals the potency of the Manuka.
Docosahexaenoic levels (DHA - is one of the omega-3 fatty acids) levels to give an indication of the shelf life ahead of this honey,
Hydroxymethylfurfural (HMF) content measurement, which signals freshness.
Leptosperin pollen count (a naturally occurring chemical, found only in the nectar of Manuka plants and a few very close relatives) which indicates authenticity.
There are many types of fake Manuka honeys in the world, some actors well known for injecting Methylglyoxal (MGO) into any old honey and trying to pass it off as Manuka and that's why if the consumer is going to invest money for a jar of honey, they should at least ensure it's a UMF recognized honey brand and that each jar it put through the world's toughest and most universally recognized analysis. Anything less and you're risking getting an inferior product.