Yes: Page 30 of The Operator's Guide for IBM 3270 Information Display Systems calls it a Tab key.

   Tab also has typamatic capability that allows you to move the cursor quickly from field to field.

  The Back Tab key moves the cursor back to the first character position in an input field. If the cursor is already in the first character position of an input field, and if you press the Back Tab key, the cursor will then move back to the first character position of the preceding input field.

So my guess is that the cursor defaults to being at the start of a field as you navigate, so Tab and Back Tab work as expected. But if you're editing a field and have moved the cursor within the field then Back Tab acts differently.

https://usermanual.wiki/Document/GA2727421OperatorsGuideforI...