Class exceptions are a type of exclusion that is not rule-based, but rather is applied to individual classes during the manual review of a sync analysis. When class exceptions are applied, they are then saved and applied on all subsequent syncs. Class exceptions created during a sync analysis review are added to a global list for all jobs, where they can be later reviewed, removed, or have notes added to them as a reminder to their purpose.
In general, for performance reasons, class exceptions should be used sparingly, as a time-saver for resolving difficult-to-determine cases while attempting to manually match classes. It is always better to revise your inclusion and exclusion rules if you find yourself applying lots of class exceptions to a sync job.