check the main program of the screen, do a search for the field in the program (use global option).
Now you will see where the value is coming from. This could be a structure as well. In that case do another search for this structure name and see where this structure is filled and by which values.
That will give you a clue. You can then place breakpoints to see which value is used IN YOUR CASE, since the field can hold values from different tables.