We don’t usually share fan speculations here, but this one is a fun discovery that seems too good to be a coincidence!
Back in 2015, one of WITH Tour VCRs showed a shelf with books on it (below). It should three volumes numbers: 15, 17, and 18. A book titled last minute, and an encyclopedia volume of No.6 for letters “HIJ”.
Back then, most fans interpreted the first 3 volume numbers as the years where Tohoshinki will be touring: 2015, 2017 and 2018, possibly hinting to them enlisting together (so no 2016), which is what happen, as well as tours were indeed carried out right after they discharged in these years.
There were not much theories about the other two volumes, some probably thought HIJ stood for a “History in Japan” release, but nothing else.
TVXQ! Express 