The most famous regions are Sanani, Harazi, and Matari. All of them are labeled “Moka” (or Mokka, Mocca, Mokha) because that is the varietal of coffee plant which Yemen grows. The only other place that grows Mokka is Ethiopia Harrar. A farmer in Guatemala managed to harvest 8 pounds of Mokka and sold it for $505/lb because of how rare it is to grow the varietal outside of the middle-east region. All three regions are known for undertones of bitter herbs and cocoa, but the Harazi is generally considered the most desirable region, and the Sanani is the most common (although no coffee from Yemen could really ever be considered "common")
None of Yemen’s coffee is sorted by size or quality, which makes it tricky to roast and tricky to purchase. It produces very high amounts of chaff, and some of the beans are tiny, while others are large. It is a natural-processed coffee and should be roasted like a natural Ethiopia. Light roasts can often bring out tastes of blueberry or orange, along with notes of bread, hops, bitter herbs, and raspberry. The darker you roast it, the more bitterness you find. At a medium roast, Yemen coffee makes fantastic espresso, but the price and rarity of it makes it impractical to use as espresso in a commercial setting.