Inclusion of a record ID distinguishes an update from an insertion. Record IDs follow the same naming convention across data feed types: fieldName.id. Each data feed type may include one or more record ID columns to identify the object to update. Refer to the specific data feed type for the record ID definition.
If you are updating a record, any fields identified in the file will be updated. If a row of data does not contain a value for the identified field, then that record’s field will be updated with a null or empty (i.e. for text fields). Any fields not defined within the file are ignored during the update.