To Create a New Variable while applying a 2D Dimensional Constraint

Select the desired Dimensional constraint tool (Home > Constraints > Dimensional).
Select the profile(s) or segments to constrain.
Enter a data point.
The following drop-down appears:
Select New Variable.
Edit the variable name and value in the text box.
Enter a data point or press <enter> to accept the new variable.

The new variable is created and applied to the constraint. This new variable is added to the variables list in the Variables dialog.