If you intend to use formal markup, first read the chapters in Part 2 - The Author's Job to become familiar with the set of HelpTag elements. Although shorthand and formal markup share the same tag set, there are several important differences.
Chapter 8, Reading the HelpTag Document Type Definition, explains key components of the Document Type Definition (DTD) and shows you how to create formal markup. The complete HelpTag Document Type Definition appears in Appendix A.
The Developer's Kit includes the HelpTag Document Type Definition. The file is located in the /usr/dt/dthelp/dthelptag/dtd directory and is named helptag.dtd.