This setting is found in the Options: Inventory section of the System Setup area. Yellow Dogs recommended best practice is to always consider UPCs globally unique. This feature is best used in scenarios where each store within Yellow Dog is managed completely separately from the other locations i.e. hospital gift shops. These are very often run by volunteers who typically only have access to the store they work at and would not be able to see or edit the UPCs being used by other stores. Leaving this box unchecked will ensure they are able to edit the items in their location without issue.
Note: Permissions are required to see this area. If you cannot see System Setup when logged in, you need to see your onsite admin for additional permissions.