For elements that are used for reference purposes, the Define attribute isn't necessary. For example, after loading client, the following example shows how a client is referenced during the creation of a child matter:
 

When referencing an existing item, the ID of the object is always used to reference the object. You need to include only the ID attribute when referencing the item. All other attributes are optional.

It's possible to define multiple objects within the same XML file—for example, a matter and the file parts within the matter. Performance is actually improved through caching if a hierarchy of items is created in one batch.