Source: http://svn.oss.backendmedia.com/library/OutputWriter.phps

The purpose of the OutputWriter is to combine the output of all the modules stage3 using a FrameTemplate. Based on the page configuration (which may have been modified by any of the loaded modules) it sets the necessary headers and pushes out the output. This can just be text or also an email based on the output setting of the page configuration.