Copy only backups were introduced in SQL Server 2005 so that DBA has an option of taking ad-hoc backups without breaking the backup sequence. In SQL Server 2005, copy only backup was available only for as a T-SQL option and in SQL Server 2008, this feature came to UI.
In SQL Server 2012, there is another UI change!!!!
Copy only backup does not have a meaning for differential backups since ad-hoc differential backup does not effect any sequence.
However, in SQL Server 2008 still you can get a copy only differential backups.
In SQL Server 2012, we you select differential backups copy only backup option will be disabled.
you can read more about copy only backups from here.