As a system-generated unique identifier, Course ID is the most reliable identifier of all for courses. By this logic, I populated my external software with Course IDs. But the Data Import for Academic Enrollment allows only Course Code (a user-generated, non-required field in the course record that the system doesn't force to be unique) or Course Title (again, the system doesn't force the title to be unique). Course ID should be an option, for it's clearly the most reliable!