Both files have the same format.

A          R VEHPHY                    PFILE(PFVEHICLE PFHISTORY)
A          K SBUYER                                             

That's a UNION.

Normally the INFDS (assuming you are using the READ opcode to read the records) will reflect the file name. I know when you do an OVRDBF xxx TOMBR(*ALL) it shows you the current member name. Perhaps the member name is the only thing updated--maybe look at that instead of the file name.

Are you stuck with this LF? You might need to create a multi-format LF:

     A          R VEHPHY                    PFILE(PFVEHICLE)
     A          K SBUYER
     A          R HSTPHY                    PFILE(PFHISTORY)
     A          K SBUYER

With a multi-format LF, you can use the INFDS to get the format name you just read (in my example, 'VEHPHY' vs. 'HSTPHY'). Once you know the format name, you know which PFILE.

Thanks for your replies, I had to lose the join file and just process one time with each physical.

Now they want to sort all blanks to the end, I told them they would have the records in reverse order.  Have I remembered incorrectly?  Is there a way to accomplish this in some other manner?

Yes you are. Creat another logical that has select/omit to include only blank records and process it last (3rd iteration).