Microsoft Knowledge Base Article 225087 contains the following summary:
This article is a guide for writing custom ADM files for use with System Policy Editor. System Policy Editor's interface, comprised of all of the books, check boxes, and text boxes you see when you open a computer or user of group policy, is created with a template. The system policy template, or ADM file, is a simple program that instructs System Policy Editor what books, check boxes, and other input controls to present to the administrator.
For more information about writing an ADM file for group policy, see the white paper ("Implementing Registry-Based Group Policy") at the following Microsoft Web site: