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.