The Differential Import will affect ONLY those classes (Roster) which are included in the file.  If the Course Section is not included in the file, it will ignore that class.

Prior to the start of the term that a Differential Import targets:
The roster for affected course sections will be cleared leaving no record ('Scheduling Error' reason), and then repopulated with the students in the Import.

After the start of the term that a Differential Import targets:
The roster for affected course sections will be cleared, and then repopulated with the students in the Import. Any student not included in the Import that had been on the roster prior will show as having 'Dropped' the course on the date of the Import.

Additionally:

  1. The above implies: if you only want to add a single student to an existing class roster, you still must include all of the other students you want to keep in the same class roster in your file.  Otherwise, it will 'drop' or 'scheduling error' any student currently in the class roster who is not included in the file.
  2. The Differential Import will take effect immediately.  So if you are in the middle of a term, and you want the differential import to take place at the beginning of the next term, you must wait until the beginning of the next term to process the differential import.
Note: Keep in mind that the Differential Import is section based, not student based. To drop a student from a section, the import file would need to include a row for each student who should remain enrolled in the section.

The import file should not include only the student who is being dropped with a row for each of their different sections and excluding the section that they're dropping (which would result in them being the only remaining student in those sections)