In certain scenarios you may not want to bring certain types of donor into your RE database. For example, you only want to bring alumni but not students in. We will be adding a more elegant way to do this in a future version of Importacular but for now this is a workaround.
In order to successfully create an RE constituent you need a Surname. So on the surname field, open the field settings and check the 'Conditionally update values' box, then select the field that determines the records you do (or don't) want to import and fill the text as per your incoming data (see below example).
Now any record without the relationship of 'alumni' won't have a surname and therefore won't be created. Obviously this will create errors in your control report so you can see all the unimported records there as a double check.