There are 2 ways to use Add Mirror:

1. The 'Create New Items' box is not checked: the items are only linked to the new screen group. An item can appear on multiple screens. This means there is only one instance of the item in the database, so if you delete or change an item in one group it will change in the other group since it is the same item record.

2. The 'Create New Items' box is checked: new, identical items are created in the new screen group. If items are deleted or changed in one group they will not be changed in the other group.