Mapper numbers in the NES 2.0 specification have 16 bits: 12 bits for the mapper number and 4 bits to distinguish mapper variants that could previously be distinguished only by the PRG ROM's hash value. But I don't recall kevtris specifying any authority for determining which NES 2.0 mapper number corresponds to which behavior, other than that $100-$1FF should be filled first. Is there any authority to define what each mapper number means?
In any case, I recommend reserving mapper numbers $Fxx (above 3840) for private use when working on new mappers used by homebrew games in development, much as Unicode reserves some characters for private use.
In any case, I recommend reserving mapper numbers $Fxx (above 3840) for private use when working on new mappers used by homebrew games in development, much as Unicode reserves some characters for private use.