If the NegLon attribute is zero, then normalized longitude values will be in the range zero to 2.pi. If NegLon is non-zero, then normalized longitude values will be in the range -pi to pi.
The default value depends on the current value of the SkyRefIs attribute, If SkyRefIs has a value of "Origin", then the default for NegLon is one, otherwise the default is zero.
AST A Library for Handling World Coordinate Systems in Astronomy