A Document Type Definition (DTD) is the specification by which XHTML documents are classified.
Each document type definition specifies what type of document an XHTML document will be.

Transitional – XHTML documents that use deprecated elements or attributes.
Frameset – XHTML documents that use frames (can also include deprecated elements or attributes like the transitional document type).

For more details check the below link:

https://vskills.in/certification/tutorial/html/dtd-document-type-declaration/