The 029 and 129 keypunches that would have been the data entry devices of choice prior to the arrival of these terminals created punched cards. The card was often divided into fields by a program drum or equivalent program card ready by the 129.
Fields could be skipped over, automatically duplicated from the contents of the prior data card, and forced to be numeric (shifted). Thus, the operator would type away without having to hold the shift key for numeric fields and without having to space over any areas of the card that were not being updated.
Since the keyboard type I have, Data Entry Keypunch, is usually associated with the same type of tasks as were done by the keypunch operators, it makes sense that it would allow similar efficiency for typing - skips, auto numeric shift, etc. This explains the special role of a numeric & protected field in forcing a skip operation.
A feature could be installed in the 3174 or other control unit, Num Lock, which would treat any numeric & unprotected key as if the operator had already held down the numeric shift key or set numeric lock. As soon as you tabbed or autoskipped into a numeric field, the terminal was in numeric mode to interpret key presses as the upper character printed on the cap.
Implementation was simple, as I already had logic in place to identify the numeric field and display NUM on the Operator Information Area (status line). I only had to implement the equivalent of Numeric Lock to force it to be shifted. And, of course, I had to remove the shift when we exited the numeric field.