When you create a Segment dealing with Applications, Bookings, Requests etc, you are asking for all results which match your criteria to be returned. This might include more than one result for the same student. Why is this important?
Well, a single Student may have more that one Application (or Booking, Request etc). So if you are asking for all Applications with a Status of 'REF', then your Segment needs to include every application with that Status, even if they are for the same Student.
For example, you could have a Segment result which includes:
Susan Smith Drama and English Literature (Application ID: 22229999)
Bob Jones Accounting (Application ID: 365998899)
Susan Smith English Literature (Application ID: 666665895)
Both of Susan's applications fit the criteria of your Segment, so they will both be included in the results.
You can then create a Campaign from that list knowing that all results are included and any email Campaign you create can include Application specific merge fields, such as the Course Name.