"Field Mark" vs. "Duplicate" (Hex 1C)

[James Lampert <jamesl@xxxxxxxxxxxxxxxxx>]

One of our customers brought to our attention an example of a flat file 
that used Hex 1C as a delimiter.

I'd never heard of Hex 1C (asterisk with overscore) being used in any 
other context than the "Dup" key (shift-insert) on a terminal, but a bit 
of research turned up references to "Field Mark" (which sounds like some 
kind of delimiter), and to Shift-Home on 3270 terminals being mapped to 
that value. I tried that keystroke on our 3489, and to my surprise, I 
got an error of "Duplicate key or Field Mark key not allowed in field." 
I tried it again in a field that I knew accepted a "Dup" keystroke, and 
while I had no trouble getting "Dup" accepted, "Shift-Home" produced the 
same message.

Does anybody here know what a "Field Mark" is, or why it shares the same 
hex value and glyph as "Dup," or what kind of field actually does accept 
it in 5250 data stream?