You can use the Measure... option to create a driven dimension or reference dimension that is based on a measurement of the part or assembly.
The value of this dimension might change if the model changes. For example, if an equation includes a measurement of the height of a part, and that height is doubled, then the equation evaluates to twice the previous value.
The Measure option is available from all four views of the Equations dialog box, and also from the Modify dialog box.
You can select the Measure... option from the flyout menu when you are in the Value/Equation column creating or editing global variables, features, equations, and dimensions. It appears in the flyout menu whenever a measurement is valid.