When editing a Host, you can select a class from the Available class list and add it to Included classes.

However, when you click on the Included classes to remove a recently added class, the class name does not reappear in the "Available Classes" list.
You have to undo your changes by reloading the page to see the class again in Available class.

Also, when a Class is removed from the Included Classes list, there is no visual clue as to which class was just removed.