It may mean two things.
1, A orginal arcade PCB that has been converted to a different game that used the same hardware
2, When you purchased the official the arcade PCB, and artwork say from Nintendo for example and converted an old arcade cabe a operator may already have to a new game.
I thought two was right and been adding my arcade PCBs. I thought I would just check before finding out after adding many my arcade games I had been putting them in the wrong section.