PHAFRAC column values
The grating ARF files created by mkgarf contain a PHAFRAC column. It is not used by any analysis tools nor is it used when modeling and fitting. It is contains diagnostic information used by the developers.
When the grating ARFs for individual CCDs are combined the dmarfadd tool simply copies the PHAFRAC column from one of the input files to the output. It does not try to combine the values in any way. This is also the case when combining positive and negative orders, and when combining responses from multiple observations.
Therefore, the PHAFRAC values may be different based on the order the files are input tool. Again, since the values are not used, this has user-visable effect.