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. To view this list, click on the Saved class exceptions icon (the 4th icon from the left).
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.