The abscesne of type is intepreted as a regular (key-value) object. The summary of rules that Orion uses to render XML for structured values can be found here: https://forge.fi-ware.org/plugins/mediawiki/wiki/fiware/index.php/Publish/Subscribe_Broker_-_Orion_Context_Broker_-_User_and_Programmers_Guide#Structured_attribute_values

I can tell you the rationale behind the rules out-of-the-record if you want. Find me at skype :)