You mean the only thing that actually defines if a country is NH or SH?
I know some people lump NH and SH based on where they play, but I don't like it. Japan is nowhere near being in the SH.
Also last year it was counted as a NH win when Japan beat us. Why do they change to SH now? Because of the Sunwolves? They haven't played more international matches against SH teams this year and have their internal league still running. In fact they haven't played any SH nations this year and have played six NH nations. Their main international tournament is in the NH (Asian Rugby Championship). Yes, they have played in the Pacific Nations Cup, but at the same time so did Canada and USA and they aren't considered SH.
Maybe at a point when Japan is actually in a SH only tournament then yes I think we can start talking about them being a part of some 'SH' group. But right now they simply aren't. One Super Rugby team doesn't make them SH.
And also, the Pacific Islands have similar latitudes of Australia, whereas Japan has almost the same, but in the other hemisphere (25.27 south vs 36.20 north)