When Updating Output Source
For AXS, XLS and MSQ file types where the Export Option is set to either <APPEND> or <UPDATE>, the target fields must be selected from the available field names found in the corresponding database/worksheet as shown below.

All available field names are listed in the drop down field list. A field can only be selected once during the export mapping process.
Various checks are also made to ensure that invalid field types are mapped together such as a Date --> Boolean etc. A message will be displayed if a situation like that is encountered.
Display and Language settings are not applicable when appending or updating a dataset.