Selection, feedback, and rendering are mutually exclusive modes of operation. Rendering is the normal, default mode during which fragments are produced by rasterization.
In selection and feedback modes, no fragments are produced; therefore, no frame buffer modification occurs. In selection mode, you can determine which primitives will be drawn into some region of a window; in feedback mode, information about primitives that will be rasterized is fed back to the application.
You select among these three modes with glRenderMode.