An R - Rez Source Code File is a file used for Mac OSX applications. It is a text file which is used by the application Rez. Res is a resource compiler for various Macintosh applications. Rez is included with the Apple Xcode development software. A file with the .r extension contains resource declarations for programs. Such declarations may include Icons, allerts, command sequences and string lists. Each R file contains a group of these resource declarations which are then compiled by Rez into the resource fork of an application file. a .R file needs t contain both the resource definitions and the type declarations in order for them to be compiled propperly and effectively.
Should you ever need to decompile the application sources, you can do so with a program called Derez. This application will divide the resource compilation into separate .R files. This application is useful for any Apple program developer and has an intuitive interface.