Collection of objects that hold the key and value pairs

            or

Object which each property name is the key and the value is derived from property value.

            or

Object of type Hash Map or Hash Table that provide the set of key and values.

As far as the normal usage for Struts specification is concerned, the attribute either holds 1) a collection, or 2) left blank. When blank, it would depend on the nested parameter descriptors for the specification. The implementation tags that could accepts parameters specifications, expect either collection or single key value pair. This is illustrated with examples below.