Which of the following is a characteristic of a non-clustered index?

A non-clustered index maintains a separate index structure that points to the actual data rows in the table, but the physical order of the rows in the table doesn't have to match the order of the index. This means you can have a non-clustered index on a table that already has a clustered index, as the physical order is determined by the clustered index and the non-clustered index simply provides a different way to access the data based on its indexed values.