The generic xs:untypedAtomic type applies to all attribute nodes and atomic values that are "untyped," i.e., have no specific type. An attribute node is untyped if it was not validated against a schema declaration. "Types, Nodes, and Atomic Values" in Chapter 11 describes how attribute nodes might come to be untyped.

An atomic value might have the type xs:untypedAtomic if it was extracted from an untyped element or attribute. Untyped atomic values can often be used wherever a typed value would. This is because every function and expression has rules for casting untyped values to an appropriate type.