Class changes are actions that are either new classes to be created or updates to be made to existing classes. During the review process, to select or deselect multiple, click on any part of the row, hold the Shift key, and select another row. All items between clicks will be selected / deselected.
Once a set of actions has been selected, you can approve and stage the actions, or exclude and stage. Either action will add selected actions to the staged actions list, in the appropriate state. At this point, the actions have not been performed, but merely staged.
In general, you will likely want to review and commit an action for all recommendations made by a sync analysis, however it is not a requirement. I.e. you can stage and commit only a subset of the actions discovered during analysis. Actions get committed from the summary tab.
Note that committing staged actions will cause the job to revert to "awaiting analysis" and a new analysis will need to be run before you can return to the review process.
Also note that navigating outside of the sync analysis when you have uncommitted, staged actions will cause staged actions to be returned to the originating list, requiring you to have to re-approve them.