External Entities are a list of other schemas that the DTD will use.
An External Entity is generally called by a XML notation (see XML notation (DTDs only)) in the DTD. An external entity is represented solely by a definition within the encyclopedia; there are no graphical symbols on a diagram that represent it.
The general steps for modeling and generating a DTD for an external entity are:
1 Create the external entity and associate it to a Schema design by specifying the name of its XML Schema within the external entity’s definition.
2 Within the external entity's definition, specify the notation that calls the external entity.
3 Make sure that the notation references the same XML Schema as the external entity.
4 Generate the DTD for an XML Diagram that references the same XML Schema as the external entity.
According to DTD specifications, the SYSTEM keyword is generally intended for private DTDs used by a single author or group.
The System field holds the location (path name or url) of the external file. For example:
http://www.somedomain.com/product/file.jpg
The DTD that is generated will contain the SYSTEM keyword followed by the specified path and file name, for example:
<!ENTITY jpeg SYSTEM 'http:\\www.unicomsi.com\products\product.jpg' NDATA jpg >
Public Name
PUBLIC is an XML keyword indicating that the DTD is intended for broad use. When the PUBLIC keyword is used, you must also specify a name for the DTD.
If you want to use the PUBLIC identifier instead of the SYSTEM identifier, fill in the Public Name property. If the system sees a value in the Public Name field, the PUBLIC identifier is used instead of the SYSTEM identifier in the generated DTD, for example, if the following properties were set for an external entity named jpeg:
<!ENTITY jpeg PUBLIC 'Product JPEG' 'http:\\www.ibm.com\products\product.jpg' NDATA jpg >
Notation
Specify the name of the XML notation (see XML notation (DTDs only)) that calls the External Entity.
Comments
Text in the Comments field will be generated into the schema file but will be formatted as shown below:
<!--your comments go here-->
Processing Instructions
Processing Instructions are passed to an application. They typically contain a declaration of the target application and whatever instructions the application needs to process the XML schema.
Description
Text in the Description field does not get generated to a DTD, but is generated to a BizTalk schema. You can enter a description of the modeling element, or its use in the schema. The description can be included in a report. By default, it is included in HTML reports.
XML Schema
Specifies one and only one schema (see What is an XML schema?) that will be used throughout the diagram, and/or the XML Schema definition that XML elements (see XML element) and attributes are keyed to. Note that all elements and element attributes created on the diagram will default to this schema name. Attribute and element symbols can be keyed to one and only one schema. If the schema is renamed, all elements and attributes that use it will be rekeyed to the new name. “Names” can be the same as long as they are keyed to different schemas.
The attribute and element instances will use the diagram's schema by default, however, this is not required. Element and attribute symbols using schema outside that of the diagram will display with dotted borders.