Most whiskey bottled and sold by a brand, regardless of whether it is labeled a “Single” - which is to say that it is the product of one distillery - as opposed to “Blended”, the product of more than one distillery - is actually a blend of a sort. Confusing? Well it needn’t be. Despite the fact that all the whiskey in the bottle has been distilled at a that one “single” distillery, it will have been matured in a variety of different barrels, before being combined, or blended, prior to bottling.
However, from time to time, a producer will bottle the contents of one barrel in a small batch release. All the bottles from the batch are thus not only from a “single” distillery but also form a “single” barrel.
Thus “Single Barrel Bourbon Whiskey”, is whiskey not only distilled and matured according to the strict US legal requirements for single distillery Bourbon whiskey, but will actually be the contents of a single barrel from that distillery. As a result it will exhibit all the unique and idiosyncratic characteristics that barrel has imparted during maturation.