A Segment in the LMS is an educational stage in your school/organisation, e.g. Primary for kids aged 7-12 and Secondary for kids aged 13-18.
A Level in the LMS is the stage in your Segment, e.g. for a Primary Segment, you can have Levels 1-6 and for Secondary you can have Levels 7-12.
A Class in the LMS is a group of Learners who are in the same Level, e.g. for Level 1, you can have Class 1A, Class 1B and Class 1C.
A Subclass is a division within a Class, e.g for English, you can divide your class into Beginner, Intermediate and Advanced groupings.