I believe there have been previous posts regarding I Class. IIRC, this used to be one of the upgrade buckets for NW, from B to I, just like Y upgrades to Z with miles. However, I Class has always been a paid fare class for KL and AF, so to keep harmony amongst the ST carriers, NW created U Class as the new upgrade bucket from B fares. I Class remains now as a paid WBC/FC fare basis for NW, just like for the other ST carriers.

While NW does not have any I fares of their own to sell, that I know of, there are a number of I fares offered by other ST airlines that require a connection to a NW flight and that require I Class to be available in order to confirm a seat in the front cabin on the connecting NW flight. I is now similar to C, Z, and D, which are all subclasses of the full J fare bucket.

While NW made this change some months ago, to try to keep harmony amongst the ST carriers, KLM still has an interesting anomaly which they have not resolved. They use D Class for both mileage award tickets in the front cabin and also for upgrades from paid B fares to the front cabin. And their yield management programming is such that they tend to severely limit the latter upgrades, while still offering plenty of seats for free to passengers who wish to claim a mileage award originating in AMS. Having purchased several B fares to cities via AMS, I speak from experience here. The AMS-XXX segment might show D9 if you originate in AMS but if you are connecting from the US to go beyond AMS it shows D0. I have not thought through the logic for this, but it appears that KLM definitely does not want to allow NW passengers to upgrade with miles on Europe Select flights beyond AMS, yet they will give the seats away for free to any member of a ST carrier's program who is willing to pay the extra miles for an award D class ticket that originates and terminates in AMS. I find this very frustrating, to say the least.