#### Description:

This attribute contains a textual representation of the physical units used to represent formatted coordinate values on a particular axis of a Frame. The astSetActiveUnit function controls how the Unit values are used.

Type:

  String.

#### Applicability

##### Frame

The default supplied by the Frame class is an empty string.

##### SkyFrame

The SkyFrame class re-defines the default Unit value (e.g. to `"` hh:mm:ss.sss`"` ) to describe the character string returned by the astFormat function when formatting coordinate values.

##### SpecFrame

The SpecFrame class re-defines the default Unit value so that it is appropriate for the current System value. See the System attribute for details. An error will be reported if an attempt is made to use an inappropriate Unit.

##### TimeFrame

The TimeFrame class re-defines the default Unit value so that it is appropriate for the current System value. See the System attribute for details. An error will be reported if an attempt is made to use an inappropriate Unit (e.g. `"` km`"` ).

##### FrameSet

The Unit attribute of a FrameSet axis is the same as that of its current Frame (as specified by the Current attribute).

#### Notes:

- This attribute described the units used when an axis value is formatted into a string using astFormat. In some cases these units may be different to those used to represent floating point axis values within application code (for instance a SkyFrame always uses radians to represent floating point axis values). The InternalUnit attribute described the units used for floating point values.

- When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies.

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