Read First: Object Type, Mapping The range of a mapping is the set of things which may be returned by the mapping as output. For example, the range of the mapping from people to their names is the type String. Some members of the range may never be returned by the mapping. Since mappings always return sets, to accommodate multiple-values and no-values, it is more exact to say that the range is the set of things which may be in the set returned by a mapping. It is an error for the mapping to return something that is not in its range. See the summary with example at Place Relation. Read First: Object Type, Mapping