There is only one ObjectTag for all classes, ie you cannot have the same ObjectTag for more than one class.
Each symbol with the same object tag should have the same class.
If somehow (after changes to tags or classes) a Symbol has a class that does not match the correstponding ObjectTag then these show up in the Object Class <> Symbol Class tab in the reviewer.
To resolve this problem, first click "Remove OTags from non included" and rebuild.
If that does not work then you may have to set the Symbols that have wrong object tags.
Note - this problem mostly occcurs with models built in earlier versions of ControlDraw