This attribute specifies how many digits of precision are
required by default when a coordinate value is formatted for a
Frame axis (e.g. using AST_FORMAT). Its value may be set either
for a Frame as a whole, or (by subscripting the attribute name
with the number of an axis) for each axis individually. Any
value set for an individual axis will over-ride the value for
the Frame as a whole.
Note that the Digits value acts only as a means of determining a
default Format string. Its effects are over-ridden if a Format
string is set explicitly for an axis.