see also Fields, DuplicateRecords
A calculated field is a normal field which has had a calculator attached.
When you attach the calculator, specifying true as the second paramter to calculateWith() will cause the calculator to only be used for default values.
Another way to set this is to call useCalculatorOnlyForDefaults() after attaching the calculator.
A default calculator is used, instead of reading the value from the field:
There is an important point here relating to user behaviour. If a default calculator is attached, it is effectively impossible to save an empty field on a new record. If the user strongly desires an empty field they can edit an existing record and remove the contents of the field.