The first column of the header row in a generic import is called the "Command Action".
Row data in this column will tell the import engine what type of import operation should be performed with the given data fields. Valid options for row data values in this column are:
Insert - uploads new records
UpdateOne - updates one (1) record if it already exists
UpsertOne - updates one (1) record if it already exists, if not inserts one (1) new record
These values are case-insensitive. UpdateOne and UpsertOne are the commands for updating. When used, they expect and require only one (1) record may be updated, and the import engine will error if there are insufficient key columns (recordIdentifier=True) to uniquely identify a single record. The commands "Update," "Upsert," are legacy commands that should never be used, because they do not have this protection (and they can damage data because they do not correctly execute post processing logic if more than one row of data is updated).
If a Custom_Field table is used, linkCF=True should be used in the Command Action Header column and the field column must be added at the end of the import file.
linkCF=True - This flag tells the import processor to scan the tables present in the import, locate custom field tables (SOME_TABLE_CF) and attempt to link them to a parent table (SOME_TABLE). For this to work the CF table must not have a recordIdentifier specified and its parent table must be present and must have a recordIdentifier. The default is False, as this scanning logic adds performance overhead to the import.