Filling involves setting the target area of the destination surface to a specifed value before transferring information to that area. This can be useful when drawing large images that contain a predominant color, initialising z-buffers, etc.
Color filling involves setting all of the target area to the same color. This can improve performance because the fill color will not need to be copied from the source.
A depth fill sets the specifed area of a z-buffer surface to the same z-value, effectively clearing the z-buffer to a given background depth.