The XML | Add Child | ExternalID command is available only when the DOCTYPE declaration of an XML file is selected in Grid View. This command inserts a Grid View row for an external identifier (PUBLIC or SYSTEM). You must enter the type of identifier and its value.
The Text View corresponding to the screenshot of the Grid View shown above looks something like this:
<!DOCTYPE OrgChart SYSTEM "orgchart.dtd" [
<!ELEMENT name (#PCDATA)>
Please note: A row for External-ID can be added as a child when the DOCTYPE item is selected, or it can be inserted or appended when one of the child items of the DOCTYPE item is selected, for example, the ELEMENT declaration name in the example above.