Your options for Import type will change depending on which module you are in.

Admission's Office
Use this Import type whenever you need to create or update information specifically on the Applicant view of a record

Registrar's Office
Use this Import type when we need to either add a new student record which does not currently have an applicant record.
This can also be used for any general updates to existing student records

Use this Import type when we enter information specific to the Enrollments tab of a student record
This can be helpful to update existing enrollments or adding a new enrollment for a previously withdrawn student

Use this Import type when adding new relationships for records or updating existing relationships.