I'm not sure why "I" fare bucket is being displayed again, as others have pointed out it used to be the upgrade bucket for WBC on B fares. Incidentally, I was "I" space showing on one of my tools last week on some transpacs and just assumed it was an error. I can't recall the routes/dates I was searching for so cannot replicate the results to check deeper.

I can tell you this on the NW transpacs, if you see J0 and then anything in the C or Z buckets, it's likely an error and there is really no space at all left to sell in WBC. I was in a back and forth with my TA last month about this because I kept asking her to book me on days that I saw C space available (J0C6Z6 etc) but she was insisting there was no space to sell. Ultimately ended up booking for the day before and waitlisting and clearing shortly thereafter. Not sure if others have similar experiences.