displayText

Text shown to the user when it cannot be derived from value on every render: the edit buffer of a number field (1. is not a number yet) or the label of the selected dropdown item (needs a query). null means "not determined" - the control fills it in itself, synchronously or with a query. That is why updateControl from outside clears this field: writing the value invalidates the text, and the clearing itself is the signal to work it out again.