Whenever you click on a dimension value (either
in Select 
mode
or Dimension 
mode).
The Dimension dialog helps you to:
Interactively modify actual dimensions of entities.
Adjust the tolerance and precision of displayed dimensions.
The Dimension dialog appears:
When you first assign a dimension.
Or
Whenever you click on a dimension value (either
in Select mode
or Dimension mode).
The Dimension dialog displays the exact value of a dimension, while the corresponding dimension displays a rounded value.
