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This functionality is found in the Format Editor>Fields.
The setting is available on each field, except Spatial fields, once the Use Special Function check box is selected, and has Concatenate in the field and a Mask text field. The Concatenate setting is used to link incoming data from other columns in the data source together and set a field's value to the result. The Mask defines the data that is linked together. This Mask can contain incoming field values from the data source as well as any other text. It is advised to place any fields using the Concatenate function at the end of the Format, to avoid conflicts with existing columns in the file being imported. If a field has the Concatenate function applied to it, there should not be a column for that field in the data source.
Rules about the Mask:
- To use the incoming data from a field in the import, place brackets  around the Incoming Key. For more information refer to Determining Incoming Key for Importing.
- Fields specified in the Mask must exist in the Field View of the Format.
- The Mask cannot reference the field it is on.
- Any other text is included in the Mask as is.
- The Mask is required if the Concatenate function is selected.