A Calculated Field, or as it also known, a Derived Field is a special type of database field that derives/ or calculated its value from other fields. An example might be a FullName field, that is calculated from FirstName and LastName fields.
With .NET derived fields are available natively. You can create a virtual property, which you tell Data Abstract to ignore so it doesn't attempt to retrieve the Data from the server. You can see an example in the .NET Todo List tutorial.
For other platforms Data Abstract provides classes that add support for calculated fields: