Have you tried pressing F1 while on that entry in the HMI design GUI?
I can only guess that it is the "Object Number" of the widget on the target screen that gets keyboard focus when that target screen is activated, so the user does not have to take an extra step to be able to e.g. start typing to enter a value into a text box widget.
This is only a guess; if there are object numbers assigned to each widget (button, shape, text box, etc.) on a screen and those are available to, then the likelihood of it being a good guess goes up.