The DIALOGS.RES and DIALOGS.H files together are an archive of all the dialog information. The .RES file contains all the dialog information except the symbolic names you've defined, which are in the .H file.
Don't confuse the .RES file written by the Dialog Editor with the .RES file produced by the Resource Compiler that is linked into your application. The .RES file that the Dialog Editor writes is just an archive for the Dialog Editor's use.
Your programming environment will thus likely have two files with an .RES extension—the Dialog Editor's, and the Resource Compiler's output. It's up to you to avoid a name conflict between the two.