Classes are defined as a list of possible arcs that a character can choose within a campaign. These are going to be dependent on the specific Tale. A tale can either afford a large portion of classes, or a small amount, it depends on the narrative. Classes offer their own unique play style in comparison to other characters because classes offer unique equipment, abilities, and items to people who choose such.
Things such as having a specific magic, being proficient in a certain skill, or being something akin to an Afflicted Being would be examples of different ‘classes’. There will be previews of Classes so that the players can get a feel of what they could possibly be picking before engaging in a Tale, however sometimes this could be left up to chance or surprise depending on the DM in question.