The creator field of your application file should contain its own signature. Most programming environments provide a simple tool for setting the creator field of your application file. Your application can create documents of any type, and it can specify any application as the creator. You could write a utility application, for example, that creates a new document by opening one text file and appending onto it another text file. The application would give the new document the same creator as the first original text file so that the Finder can call on that application when the user wants to open or print the new document.
Most word processors allow the user to create text-only files.
A document of file type 'TEXT' can be opened or printed by any application that accepts such file types. Your application can still assign its own signature as the file's creator so that the Finder can call on it to open or print the file when appropriate. Because the document's file type is stored in the catalog file and the Finder stores a list of your application's supported file types in the desktop database, the Finder can determine whether to launch your application. If the document's file type is supported by your application, the Finder launches your application and passes it the name of the document. For example, if your application is a page-layout program, it might create documents of its own file type while also supporting documents of 'TEXT' and 'PICT' file types.