When writing a FrameSet to a FitsChan, each CTYPEi value is set to the value of the Symbol attribute of the corresponding axis in the Frame being written. If a value has been set explicitly for the axis Label attribute, it is used as the axis comment (except that any existing comments in the FitsChan take precedence if the keyword value has not changed). The above procedure is over-ridden if the Frame is a SkyFrameSkyFrame or a SpecFrameSpecFrame, in which case the CTYPEi value is derived from the SystemSystem attribute of the Frame and the nature of the pixel to WCS MappingMapping according to the conventions of papers II and III, as described below.