MongoDB collections automatically include the _id field, and the driver, by default, maps this field as an _ID column. Because some third party applications are unable to process identifiers with a leading underscore, you may need to replace this leading underscore. The LeadingUnderscoreReplacement configuration option allows you to replace leading underscores with a string. For example, by specifying LeadingUnderscoreReplacement=XX, the _id field becomes the XXID column in the relational view of your data. In addition, any other fields or collections with a leading underscore would be modified in the same manner.

For details, see "Identifiers" and "ConfigOptions" in the driver documentation.