Storing an XML Document/Fragment as a DOM object is expensive both in terms of memory and processing power, for this reason when the client receives XML/data from the server, it is initially stored as a String.
If and when the API user calls the getContentAsDOM() method in XMLResource the String is then serialised into a Document or Node Object depending on the current config.
If and when the API user calls the getContentAsSAX(ContentHandler handler) method in XMLResource the String is piped through the ContentHandler as a set of SAX events.