The MOC courses are NOT linked to the certification exams. Each course is designed to prepare you for a specific task, which might not necessarily map to a specific exam that tests only for that indiidual
task. So, preparing for one exam may involve taking several courses as well as doing individual study for those parts that are not covered by any course. And conversely, taking a course may provide you with different pieces of knowledge which may be useful
for several different exams.
As to the "A", "B", "C" and so on at the end of the course numbers, they refer to the revisions of the courses, so that a "C" course is a newer version of the "B" one.