This attribute is a boolean value which controls how a SpecFrameSpecFrame
behaves when it is used (by
astFindFrameastFindFrame or astConvertastConvert) as a template to match another (target)
SpecFrame. It determines whether alignment occurs between the offset
values defined by the current value of the SpecOffset attribute, or
between the corresponding absolute spectral values.
The default value of zero results in the two SpecFrames being aligned
so that a given absolute spectral value in one is mapped to the same
absolute value in the other. A non-zero value results in the SpecFrames
being aligned so that a given offset value in one is mapped to the same
offset value in the other.