On Sumdog, classes are used to organize your students. This makes it easier to monitor your students and quicker to set activities.
Students can only be in one class, but they can be in multiple groups. So, If you teacher a class made up of mixed abilities, you can create groups for the different abilities.
Classes can be assigned to multiple teachers. When you are a teacher of a class you will be able to monitor their activity and focus their learning with activities.
If you have a class with different abilities/levels. Set up a class for all the students and then create separate groups to organize students into smaller sets reflecting abilities. This lets you manage the students logins and subscriptions.