This messages informs you that you can convert plain text files, XML and HTML files that you want to add to your project into DTX files. If you convert your files into DTX files, you can use the Doc-O-Matic Topic Editor to add content to your files. If you add plain text files, XML and HTML files, you can only preview them in the Topic Preview and export them to one of the output formats. You need to add content using an external editor. If you convert your files, the files themselves are not added to the project, only the transformed DTX files are added to the project. You do not loose your files or their formatting.