A series is a concrete and described soil: its sequence of horizons, its depth, its texture, its drainage. It has its own name—almost always that of a nearby place or ranch—and that name identifies it soil wherever soil survey map it appears.
It is the finest level of classification. A map unit is described by saying what series it is made up of and in what proportion: "series A 60%, series B 30%, inclusions 10%."
For those who buy, the series is what allows two distant propertiess to be compared: if they both have the same dominant series, they have the same soil, even if they are two hundred kilometers away.