IMHO, I think it is similar to the X mode of my Contax 645. The highest flash sync shutter speed of C645 is 1/125. When it is set to X mode, the shutter speed will set to 1/90. It just want to make sure that the C645 can work with some of the older and/or larger output studio flash systems (where 1/125th is to quick).
I guess it also applies to the N1.
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