Among coffee-producing countries, Sumatra is a remarkable sort of outlier: Though the Pacific island came to grow coffee because of European influence like much of the rest of the "New World," the coffees produced in this region are special and specific to the terroir, traditions, and culture of Sumatra.
The majority of coffee produced in Indonesia is processed in a manner often called "wet-hulled" or "semi-washed," and occasionally referred to by the Bahasa term giling basah. Coffee treated in this manner are often depulped on a farm after picking, then overnight-fermented in tanks, sacks, or other containers in order to soften the mucilage layer, which is washed off; because of the sort of "pile fermentation" style of preparation employed, the parchment layer remains wet, and is hulled, or removed, while they are still at a high moisture content.
This process is directly responsible for the classic flavors people love in a Sumatran coffee: earthy, smoky, meaty, savory, and bold. Clean cups are especially valuable, and the bass notes of that Indonesian profile can be nicely complemented by some sparkling acidity.