enumeration « XML Schema « XML Tutorial

3.66.1.Specifying a Set of Acceptable Values
3.66.2.enumeration Constrains the value of the data type to a defined set of values
3.66.3.Enumeration limits a value space to a specific set of values - if a value isn't specified in the set in the schema, it isn't valid.
3.66.4.Enumerating A Simple Type