INES Mapper 006
Mapper 6 is used for FFE disk images. That is, for ROM-images stored on floppy disks as used by old copiers from Front Fareast. Unlike normal ROM-images, these files are patched to work with the copier's mapping hardware. Portions of these patches are typically stored in the so-called "trainer" area of iNES files (the iNES "trainer" apparently being poor Japanese-to-English translation for "patch").
FCEUX ostensibly emulates this. As do most NES emulators.