A hierarchy facet has a value attribute like the enumeration facet. In addition, a hierarchy element can contain a number of hierarchy elements as children. The top level hierarchy element constrains the first element of the list. The hierarchy facet has the advantage that lists can be constrained more specific to a special group of values. All repetitions of common prefixes can be avoided.
Implementations can store elements of such lists more efficiently.
I tried to embed the hierarchy facet into the XML scheme part 2 in order to give a clear cut definition. A hierarchy is a root node. When a datatype is derived from a list datatype, also the hierarchy constraining facet can be used. The XML representation for a hierarchy schema component is a hierarchy element information item. A hierarchy which is the root node build up from the hierarchy element item appearing as a child of the simple type.