A check mark next to the Automatic Update tool means that the auto update function is active. With this functionality enabled, Impact applies any change to a variable or text entity automatically after any variable change. For example, simply click on the number to the right of the desired variable in the Variables window to change the value or measurement, then hit the Enter Key on your keyboard to apply.
When Automatic Update is not enabled then Recompute Design must be selected or the Recompute button at the bottom of the Variables Window needs to be clicked for any changes to take effect.
NOTE: You can also enable Auto Update by clicking on the Auto Update button to the left of the Recompute Button located at the bottom of the Variables Window in the Impact Explorer.