<xs:element name="ExternalFile" type="ExternalFileReferenceType"><xs:annotation><xs:documentation>The ExternalFile element gives information about an external image instruction file.</xs:documentation></xs:annotation></xs:element>
The id attribute is the QIF id of the work instruction, used for referencing.
Source
<xs:complexType name="ImageInstructionType"><xs:annotation><xs:documentation>The ImageInstructionType defines an external file that provides work instructions in an image.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="WorkInstructionBaseType"><xs:sequence><xs:element name="ExternalFile" type="ExternalFileReferenceType"><xs:annotation><xs:documentation>The ExternalFile element gives information about an external image instruction file.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>