For example in a timefield if force selection is off and the inputted text is not part of the dropdown list. the returned value is null.
It does not even try to parse the rawvalue to see if it contained a valid time.
Another example is what gets displayed on screen and whats the value of it currently. For example if forceselection is on. If getValue is not null it does not rewrite the rawvalue on the screen. However the value does not get set to null if forceselection is on and uses the old force value. So eventhough the value stored is still correct. The display on screen does not write the correct thing.