I always presumed that the "additional passport" line was for additional US passports. I know early on, I was not able to add my other US passports to the system and could only register one; but a few years ago the program changed to allow official, and second ordinary passports (and diplomatic too I presume) to be added which I did. It was very helpful to me to have the additional US passports in the system.

Interesting to know it might now cover foreign ones also.