Weird because on the mainframe 3270 the tab key was used to move between fields, at least that is my recollection.
That's exactly my memory too, using a tab was second nature and it irked me when I went to gui apps with a mouse and the tab order was wrong, mainly with visual basic apps.
It's been ages since I used an IBM green screen but I seem to recall that in our application Tab did move between fields, but I think there were also function keys reserved for this purpose.
I wonder about the midrange systems. I never used AS/400's, but I know they have the concept of a a "Field Exit" key separate from <ENTER>.
+1, always thought it was an IBM standard, move with TAB and commit the form with ENTER.