The XFA language supports fields which are calculated yet still overridable by the person filling the form. For instance, you may have an invoice on which you calculate the tax associated with a purchase using a calculated field yet, for customers from other countries, you would like to allow them to override the calculated tax amount and enter the correct amount themselves.
To do this, you would use the Object palette’s Value tab and set the field’s Type property to _Calculated – User Can Override_. You could even go as far as specifying an Override Message which would appear if the user attempted to override the field’s calculated value.
In theory, when the user attempts to override the field’s calculated value, either the Override Message or, if none was specified, an Acrobat default message appears warning that the field is calculated and gives the user the option to continue with the override or to cancel. If the user proceeds with the override, subsequent calculations use the override value instead of the original calculated value regardless of subsequent changes to other fields which the overridden field’s calculations were dependent on.
Unfortunately, there’s a bug in Acrobat that causes the overridden value to be discarded. It manifests itself in a couple of different ways:
# *Dynamic PDF:* Not only will the value specified by the user (the override) not be retained but the Override Message (if specified) won’t be displayed. The behavior is simply to let the user type-in a value but then throw it away and run the calculation again without any warning.
# *Static PDF:* In this case, the Override Message (if specified) is displayed and the user has the option to discard their change or proceed with the override but the specified value is discarded regardless of what they choose to do and the calculation is run again.