Because grandma liked soaking plums in slivovitz.
General notes here. General notes here. General notes here. General notes here. General notes here.
Sugar: The Slivovitz goes inside the barrel.
Prunus Intactus Technically, it's fruit preserved in alcohol, especially when the recipe doesn't actually include Slivovitz plum brandy. It's common to see recipes that recommend vodka. We would disapprove, but since it actually tastes pretty good we're going to let it go.
See Cathy Barrow's recipe in the Washington Post. for an example of Prunus Intactus.
Other flavor agents: additives!
List of Sweet Plum BrandyCastle Spirits - Spirit of Solomon
link to bottle page