Keyboard combinations Left ALT-030 and Left ALT-031 (numbers must be typed from the numeric keypad) represent two control (non-printable) characters from the ASCII table (record separator RS and unit separator US respectively), which are recognized by Revit as characters, but are not visible. We can use those characters to “cheat” Revit when it requires inserting alphabetical character(s), but the user would not like any character to be visible – like in the case of manual overriding a dimension value measured from the model or drawing.
The above control characters can also be inserted using a contextual menu revealed with right-click in any textual field. This is often more convenient than typing the “Left ALT-“combinations, especially on some laptops, which cannot even simulate the numeric keypad.
Trying to overwrite the dimension number manually causes “Invalid Dimension Value” message
Using Left ALT-030 before the dimension number allows overriding it with a different value