In addition to setting metadata about an image, you can also set metadata about individual frames in an image. This is most useful when you are making a set of images to distribute. The frame metadata will then be used either in the Icon Container file with the set of images or in the individual images from the frames when exported.
To change between editing image and frame metadata, use the switcher at the top of the metadata section of the image inspector (this switcher will not be visible if there is only one frame in the image, in which case the metadata for the image and frame are the same).