In Teacher Dashboard, Subjects are used to group assignments and quick share files together so they are easy to find within a student's OneDrive. Subjects are just a list of selectable options rather than a group of users, and therefore you could teach any number of classes for each subject, as well as any number of subjects for each class.

Visit Administrator instructions on adding new Subjects

Some teachers may not want to see all of the subjects that are available across the entire school and they want to create a list of 'favorite subjects' only.