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aRA - Radius value
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The radius value is used by rotational axes only (A, B and C) to convert linear units
to degrees when in radius mode.

For example; if the A radius is set to 10 mm it means that a value of 62.8318531 mm 
will make the A axis travel one full revolution - as 62.383... is the circumference 
of the circle of radius R ( 2PIR, or 10 * 2 * 3.14159...) (Assuming $nTR = 360 -- 
see note below). Receiving the gcode block G0 A62.83 will turn the A axis one full 
revolution (360 degrees) from a starting position of 0. All internal computations and 
settings are still in degrees - it's just that gcode units received for the axis are 
converted to degrees using the specified radius.

Note that the Travel per Revolution value (1TR) is used but unaffected in radius mode.
The degrees per revolution still applies, it's just that the degrees were computed based
on the radius and the Gcode axis values. See Travel per Revolution (See 1TR) in the 
motor group. 