A DOCTYPE declaration in an XML file can contain a reference to an external resource containing DTD declarations. This resource is referenced either through a public or system identifier. For example:
<!DOCTYPE doc_element_name PUBLIC "publicID" "systemID">
<!DOCTYPE doc_element_name SYSTEM "systemID">
A system identifier is a URI that identifies the external resource. A public identifier is location-independent and can be used to dereference the location of an external resource. For example, in your XMLSpy installation, URIs for popular DTDs and XML Schemas are listed in the catalog files named catalog.xml in the various schema folders in C:\Program Files\Altova\Common2020\Schemas\. A public identifier in an XML document can be used to dereference a DTD listed in these catalog files.
The XML | Append | ExternalID command is available when a "child" item of the DOCTYPE declaration in 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.