Impact gives you the ability to override a value calculated by an expression. In this case it will highlight the value indicating that it is no longer the result of a formula but a hard number entered by the user.
The Reset option will return a selected expression's overridden value back to the sum of the original defined calculation.
To use just right click on the expression and select Reset.
This option is only available if the value has been overridden and if the option Use formula as default has been switched off in the Standards Designer.