XPath Prefix Mapping
To define default mappings between prefixes and namespace URIs go to the XPath preferences page and enter the mappings in the Default prefix-namespace mappings table. The same preferences panel allows you to configure the default namespace used in XPath 2.0 expressions.
Important
If you define a default namespace, Oxygen XML Editor plugin binds this
               namespace to the first free prefix from the list: 
         default,
               default1, default2, and so on. For example, if you define
               the default namespace xmlns="something" and the prefix
               default is not associated with another namespace, you can match tags
               without prefix in an XPath expression typed in the XPath toolbar by using the prefix
               default. To find all the level elements when you define a
               default namespace in the root element, use this expression: //default:level
               in the XPath toolbar.
            