A calculated field obtains its value based on the data in other fields on your
form. After you enter values in the fields that participate in the calculation,
the calculated field automatically fills with the calculation result. For example,
suppose your form has a calculated field called 'Total.' Behind the scenes,
this field gets its value from totaling the values in the 'Food Expenses' and
'Travel Expenses' fields. Each time you change a value in the 'Food Expenses'
or 'Travel Expenses' field, the information is recalculated and a new value
is automatically displayed in the 'Total' field.
As explained in About filling out forms,
the form author can prevent you from changing the value of any field (including
a calculated field) by making the field display-only. In most cases, form authors
set calculated fields to display-only. However, if a calculated field is not
display-only, you can tab to it and change its value as you can for any other
field. For example, if you have an invoice with a calculated field called Discount,
you might want to give a customer more than the calculated discount. If the
field is not display-only, you can override the calculated value by entering
a different value.