One advantage about AutoCAD dimension is that we can put in a cheat value or sometimes we just put "NTS" for Not To Scale measurement. So how do Revit handles this then?
From Revit 2009 onwards, you can actually do that though not exactly the same. Revit 2009 allows to manually overwrite the automatic dimension value with another, descriptive text.
When you double-click on the dimension text, the "Dimension text" dialog is displayed. There you can enter the overrides - the text, prefix, postfix, etc.:
Revit will warn you if you try to overwrite the measured value with numeric value - only texts are allowed.
If you really want to enter a "fake" value, you may enclose it with parentheses or prefix it with dashes.