Unique Manuka Factor or commonly known as UMF is the only worldwide standard in identifying and measuring the antibacterial strength or quality of some strains of Manuka (UMF Honey Association). It is a guarantee that the honey being sold has the special UMF antibacterial property and a UMF rating of 10 is the minimum recognised. Ordinary Manuka has only the hydrogen peroxide antibacterial property which is common to most types of honey, whereas UMF Manuka has both the natural hydrogen peroxide antibacterial property and its own natural UMF antibacterial property, giving it increased antibacterial potency. The UMF property is very stable, unlike the hydrogen peroxide antibacterial property common in most honey which is easily destroyed by heat, light as well as certain enzymes in body serum.
UMF Manuka, also known as "Medihoney" in some pharmacies, is the preferred honey for wound dressing and other special therapeutic uses and studies are showing Manuka honey with high levels of UMF could be very effective in helping relieve stomach ulcer symptoms and gastritis, and sore throats, and when applied topically, in assisting the natural cure of skin ulcers, wounds, burns, boils, cracked skin. That is also why many skincare products also contain UMF manuka as a special ingredient and promise positive Mnauka honey benefits from their regular application on the skin.
UMF is not found in the nectar of all Manuka flowers, which are known as Leptospermum scoparium and belong to the the Tea Tree bushes found in New Zealand's coastal areas. (To be more accurate here, Manuka is also found in Tasmania, but it has been so successfully marketed and branded by New Zealand producers that most people see it an exclusively Kiwi product.) Some Manuka bushes do not produce honey with the UMF property every year, and the concentrations of UMF can vary from batch to batch and year to year. The reason why only some Manuka honeys have the unique UMF antibacterial property is not yet known. Researchers believe that it could be from a subspecies of Manuka or due to some environmental factor such as soil type.
Full account in: https://www.benefits-of-honey.com/manuka-honey.html