2.2.5.2.1.1 RefId Attribute

All <Obj> elements representing Complex Objects (see section 2.2.5.2.2) SHOULD have an optional RefId attribute that identifies the object so that it can be referenced later. The object identifier used MUST be unique during the lifetime of a serializer/deserializer pair (see the following section for details). The identifier can be any string that is valid in an XML attribute.