Data Loader behavior with Bulk API enabled using 'Insert null values'

Enabling the Bulk API in Data Loader allows you to load or delete a large number of records faster than using the default SOAP-based API. However, there are important differences in behavior when the Bulk API is enabled. The following settings are not available on the Settings page in Data Loader when the Use Bulk API option is selected See Configuring Data Loader : 

Insert Null Values Behavior with Bulk API

The Insert null values option enables Data Loader to insert blank mapped values as null values during data operations when the Bulk API is disabled. When the Bulk API is enabled, empty field values in your CSV or data file are ignored entirely — the existing field value is preserved. To explicitly set a field value to null when using the Bulk API, enter the special value #N/A in that field column in your data file.

Insert null value to a field with the Data Import Wizard

Allow Field Truncation Behavior with Bulk API

The Allow field truncation option directs Data Loader to truncate data for certain field types when the Bulk API is disabled. When the Bulk API is enabled, if a value is specified that is too large for the target field, the load operation fails for that specific row rather than truncating the data. Review and clean your data file to ensure field values do not exceed the target field's maximum length before loading.

Data Loader Behavior with Bulk API Enabled