DTD - External DTD

External DTD is a type of Document Type Definition that is stored in a separate file from the XML document. The XML document refers to this DTD file using the <!DOCTYPE> declaration.

The main purpose of an external DTD is to define the structure, elements, and attributes of XML documents in a centralized manner. This allows the same DTD to be used by multiple XML files.

External DTD greatly improves reusability. When several XML documents follow the same format, they can all be validated against a single DTD file.

Another advantage of external DTD is easy maintenance. If the structure needs to be changed, only the DTD file is updated, not every XML document.

External DTD is especially useful for large and complex applications, where consistency across documents is essential.

During validation, the XML parser loads the external DTD and checks whether the XML document follows its rules.

In summary, external DTD promotes standardization, scalability, and efficient management of XML documents and is preferred in professional and enterprise-level applications.