What is the difference between classes vs regions in CMISS?

Well, this is a fairly mirky area. People generally agree that you define a separate class for each type of problem you are solving. Eg: 3D finite elasticity is different to 2D membrane theory. So you need two classes.

Now, a region is meant to be two distinct regions in space. I think that this definition has become mirky in the past because people have used these data structures in ways that help solve their problems. If someone who knows more about this stuff is reading this wiki, please feel free to add/change things I have said. Hopefully this will help people in the long run.

A good explanation of the difference between classes and regions can be found by searching for "BIDOMAIN ITERATION" in the 'cm Lookup Frontend'. I found this to provide a crystal clear distinction between the two.