* more AA6YQ comments below
thanks for the useful comments and suggestions. I wonder why you do not recommend using the CLASS ADIF field for the DX station's FD class, it seems specifically for that purpose.
* You're correct, Bill; the QSO's class should be exported to the CLASS ADIF field.
For FD no report is received, so I assume your recommendation is to leave that ADIF field blank. Given that the ADIF specification says that specific ADIF fields override SRX_STRING and STX_STRING, it would seem unwise to populate SRX_STRING for FD when there are specific fields CLASS and ARRL_SECT that would override.
* I see no harm in always populating SRX_STRING and STX_STRING with the received and transmitted exchanges, respectively.