Sorry... It is The Difference between "FRAME-VALUE" and "SCREEN-VALUE"?
The FRAME-VALUE function returns the value of the input field that the cursor is in or was in - but you have no way of telling it which field you want the value for.
SCREEN-VALUE returns the character string that is stored in the screen buffer, of a specified object (it is an attribute after all).
IMO, FRAME-VALUE has no place in modern programming.