it's a relationship between units (classes) and a plan study.

a plan study describes the course path through the 1st, 2nd, 3rd year, ...

and, you're right at the end, i don't distinguish the course, because the offering is the same for the all the courses where she's offered.

when i add the year of lecture (id_C) is where i describe the properties of the unit, teaching date range, teachers, bibliography, .. all this is the same for every course at a given year.