Major cracks are large cracks all the way through the tub sidewall or bottom. They may not exactly be bigger, but it is more a matter of the location that will make the repair more extensive. They are normally located in high stress/ high load areas and will require more extensive work to make a repair that will be structurally sound. The repair will definitely require a full 24 hour s to fully cure before the tub or shower will be usable again. Sometimes large cracks can be the result of large/ heavy impacts or small cracks that are left to grow. Sometimes the crack can simply be repaired like a minor crack, but if it is the result of improper installation - you made to do a full tub floor "rebuild" to address the root cause and avoid the crack reappearing in the future.