For example if you have a data refresh file with Columns that looks like this:
Jane Smith F Noemail@email.com
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 firstname.lastname@example.org
John Smith M email@example.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