Specialised Frames (SkyFrame, SpecFrame, etc.) re-define the default Unit values to be appropriate to the particular co-ordinate system being represented.
For most classes, the Unit attribute is a purely descriptive comment intended for human readers and makes no difference to the operation of the software. However, there are some classes that have active Unit attributes. Changing the Unit attribute for such classes will result in the Mappings within the WCS FrameSet being modified in order to reflect the change in units. By default, only SpecFrames have an active Unit attribute.
In general, the syntax of the Unit attribute should follow the recommendations made in the FITS standard (see the paper ``Representation of world coordinates in FITS'' by Greisen & Calabretta (available at http://www.cv.nrao.edu/fits/documents/wcs/wcs.html).
KAPPA --- Kernel Application Package