Quoting Jason van Gumster :

I've been working my way through explaining GAP's video encoding feature and stumbled across something that I couldn't find any obvious documentation to (or figure out on my via trial and error). Basically, I'd like to know the difference between the SINGLEFRAMES and RAWFRAMES encoder options. The names seem to indicate that RAWFRAMES doesn't compress frame data, but if you set your output format to JPEG, the compression window appears and the encoder happily compresses each frame just as if you'd chosen SINGLEFRAMES.

I'm not certain about this, but I believe that RAWFRAMES is only meaningful for the "Storyboard" Input Mode, and then only if the source video file includes valid JPEG frames in its format (i.e., MPEG1 I-frames, or MJPEG). For these formats, rather than decode the (JPEG) frame into an image and then encode that image back to a JPEG file, the video encoder will simply extract the JPEG frame from the video stream. This is beneficial from both a speed and quality perspective.

If you are not in "Storyboard" Input Mode, encoding is mandated regardless of whether you use RAWFRAMES or SINGLEFRAMES.