RED Digital Cinema Cameras are Multi-Format/Multi-Resolution systems. Each camera has specific recording Formats and Resolutions that will vary depending on the camera body's specifications, sensor size, and sensor technology. When shooting at different Formats and Resolutions you are cropping in (or windowing) part of the full sensor size.
REDFormatKey will create a Datasheet with all of the available Formats and Resolutions for a given camera model as well as showing the maximum frame rate, image plane dimensions, image circle, megapixel count, and relative crop factors for Super 35mm and Full Frame 35mm Format sizes.
With a variety of different RED camera models working on productions worldwide this tool should be very useful for editors, post houses, and VFX studios to gather more information about the material they are working with.