Gets the export values for this field.
Choice fields such as Combo boxes and Lists have a value
selected from one of their Options. However it is possible to
specify an export value which is different from the value of field.
The export value will be used if the form data is exported or
submitted in some way so it is purely for external use. This
function allows you to get the export values from the field.
Please note that export values are only meaningful for fields of
FieldType.Combo or FieldType.List and they are only relevant for