The insert where not exists option allows you to import only those students who are new to the roster. When using this option, you do not have to include students who are already on the roster.

The differential update clears out all existing roster members and inserts only members who are in the Academic Enrollment data import file. 
Note: For differential updates, all enrolled students for each section in the file must be included in the file. Partial section enrollments will result in a drop for those students who not included in the file.