The Course Offering Code is the "organization code name" for the course. Imagine, for example, that you have a course named "An Introduction To English Literature", and "ENGL 101" as the code that your organization uses to refer to the course.
While this might be unique right now to refer to the course (the Winter semester in 2014), the course might be offered again (almost certainly will be) in a future semester. Some organizations might use the same org unit over and over and over again for each offering of a course. But most do not: each offering of a course is an entity in the system, so there's a "copy" of ENGL 101 for this semester, and a copy for next semester, and one following that. Each one has the organization code ENGL 101
but you have three offerings that the system must track.
The Org Unit Id
is the unique key to track each unique instance of an organizational unit (in this case, a course offering).
Again, most clients have course offerings associated with Semester
org unit types, or some similar "time-based" org unit grouping: in this case, when you're searching for duplicates, you might be best advised to think in terms of "duplicates right now", or "duplicate org units that are descended from the 'right-now' Semester
org unit".
Another way that clients handle things is they have a course offering template for ENGL-101, and each offering of the course inherits from that template. In this case, you might want to do duplicate checking among course offering template, rather than course offering, org unit types.