Position Comment, APRS Position, FTM-400, Yaesu
FTM-400 / Position Comment - Custom... not really On the Yaesu FTM-400 and similarly (but not identical) in the FTM-100, there are options in the Position Comment that are called Custom 1, 2, 3, etc. So, while they are "Custom" in name, they are not something you can customize either through the programmer or in the radio.

It seems that when the radio was developed, there were six spaces available in the field that stores the Position comment. Rather than leave these as empty spaces for future use, the original radio developers named them Custom 0 - Custom 6 in anticipation of future use (it helped fill the buttons on the radio in that part of the selection process too).

If you access one of these on the screen of the radio, you will find that you are not offered any editing options for it. It will appear as Custom # in the Position Comment option of APRS.

So, what it boils down to is this was a poor choice of labels in the radio. The programmer simply reflects those labels.