Data Refresh is used to bulk insert new users as well as update existing users.
When a school first engages with Blackbaud, they will submit files exported from their source system for initial import into Blackbaud. From those files, the school's Data Refresh routines are created. Thereafter, any data refresh performed will need to use the exact same column headers, spelled exactly the same (order of column headers is immaterial), with no new column headers added nor any expected columns omitted.
What a data refresh type will expect for column headers is detailed in the data refresh type's "File Layout Help" link. And, if more than one file is required in order to perform a data refresh, this will be identified in the File Layout Help.