For example if you have a data refresh file with Columns that looks like this:
                       Gender                        Email
Jane Smith      F                                 Noemail@email.com

John Smith 

Then any user that has nothing in the Columns, such as John Smith's Gender and email would be overwritten with a Blank Field.

If you have a  Data Refresh File with Columns that looks like this:
 

                       Gender                        Email                              Enroll Date
Jane Smith      F                             noemail1@email.com

John Smith      M                            noemail2@email.com

Then those whole blank Columns will just be omitted in the Data Refresh and that Data would remain unchanged. Such as Jane and John's Enrollment Dates would not be changed

Note: Usernames do not get overwritten by blank fields. Will data refresh change or delete an existing user's login credentials?