It's very confusing! This may help
http://www.bmw.co.uk/images/brochures/BMW_X3_New.pdf
This suggests you can add 6NF to 6NK IMO BUT it also says you can add 6NF to ZXF (pro media) which is strange as I have pro media but don't need option 6NF in order to use an iPhone snap-in adaptor.