If you have a file you are importing, it can contain custom fields as long as each subscriber appears on their own row and the first row contains the column headings.
So you might have a comma separated value (CSV) text file where the first row is email, firstname, lastname and the next might firstname.lastname@example.org,phil,hollows etc.
If the “define custom fields” box is OFF, only the email addresses will be imported – other values will be ignored.
If the box is ON, FeedBlitz will ask you to map the columns to any existing custom fields, or whether you want to define new custom fields with the file. Custom fields will be defined as simple text fields when created via import. If you want to import data into a custom field of a different type, you should define the custom fields first at Newsletters | Settings | Custom Fields and then map the imported custom fields in the file to the ones you created manually at FeedBlitz.