This read-only attribute contains a textual representation of the physical units used
to represent unformatted (i.e. floating point) values on a particular axis of a
Frame,
typically handled internally within application code. In most cases, the value of the
InternalUnit attribute will be the same as Unit attribute (i.e. formatted and
unformatted axis values will normally use the same system of units). The main exception
to this is the
SkyFrame class, which represents unformatted axis values in
radians, regardless of the current setting of the Unit attribute.