|Cross-referencing / Cross-referencing topics and external resources|
The scope attribute of the cross-reference (xref) is used to nominate whether the target topic, file or other resource is external to the DITA source topics. Some publishing tools (such as the DITA Open Toolkit) will generate cross-references with an external scope attribute as hyperlinks that will open the target in a new window.
If a topic being referenced in an xref is external to the document content, the xref element's scope attribute should be set to external to semantically identify this relationship.
<xref href="..../somefile.html" format="html" scope="external" />