People often ask me:-

I want to hide the value of a dimension and replace it with some other text, so I edit the dimension text and change the @D to @O (letter O), but I cannot get it to work. I have done this in the past, but the real dimension value will not be hidden.

This is a common mistake.  This technique only works with created dimensions, not with (shown) driving dimensions.  Users often forget that.  It is documented clearly in the help.  Hide the driving dimension and create a new driven dimension in its place.

Alternative technique (with thanks to Keith Jones of Cummins):-
This can be achieved with parametric and created dimensions by changing the @D to @S. Then change the name of the dimension in the bottom left of the dialogue box to the required text, eg X or B. The @S shows the symbolic representation of the dimension as seen when switch dims is used.