On Sumdog, classes are used to organize your students. This makes it easier for you to monitor your students and makes it quicker to set activities.
Classes can belong to multiple teachers too. When you are the teacher of a class you will be able to monitor their activity and focus their learning with activities.
Example: You have a class with different abilities. 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 your students logins and subscriptions.