Not all Manuka honey is equal and several rating systems are in operation. The preferred system in Australia involves testing and declaring the methylglyoxal content. Expect to see the minimum MGO rating on the label. If a Manuka honey is unrated it usually means that it has low antibacterial activity and is a good general purpose honey with some therapeutic value. A honey with an MGO rating of 190+ or higher would be considered to have high activity and 100+, moderate activity. Expect the MGO level to rise over time hence there is usually no use-by date on Manuka honey (or any honey) unless labelling laws specifically require it. The price of Manuka honey is usually directly related to its activity or MGO content.
Refer to this article to understand more about UMF vs MGO Ratings.