What does "overwrite values" mean on subscriber import?

Say you have already imported an email address into FeedBlitz, phil@feedblitz.com, along with two custom fields, FirstName and LastName, which contain Phil and Hollows respectively.

Now say you have a new import file, again containing these three fields and again containing a row for phil@feedblitz.com, except that this time the FirstName field is Philip instead of Phil.
If you run the import with overwrite checked OFF, FeedBlitz will NOT replace “Phil” with “Philip” – and vice versa.  So re-importing addresses with overwrite on is a great way to update custom fields for subscribers already in the system (FeedBlitz will not re-notify existing subscribers to a list when the import runs).