External Data Reference Field

I am wanting to use the Qualtrics standard field, External Data Reference, to store a unique ID outside of the email address. In the contact directory, the field name appears as External data reference (lower case d an r). In the survey data exports, the field name appears as External Data Reference (upper case D and R). What is the correct field name?
Also, the data source used to provide the distribution file does not permit spaces in column names. So that we can avoid from having to manually adjust the file name before we load it up, is there an API field name anyone is aware of for External data reference (such as ExternalDataReference or External_Data_Reference)?

Thanks!